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SCIENCE. 



[Vol. XXI. No. 539 



nearly horizontal suturp, thus forming a strong suborbital bony 

 wall. 



In t.be Delphinoidea, the delicate character of the suborbital 

 process of the jugal, and its union with the squamosal, render it 

 difficult at f]rst sight to determine its relation to the arch, and 

 yet, when compared with that of the horse, its homological char- 

 acter cannot be disputed. 



In the Balaenoidea, much the same conditions are presented, 

 except that the suborbital process of the jugal is both stronger 

 and more curved. The small capacity of the temporal region, as 

 well as the limited extent of the arch in the Cetacea, are corre- 

 lated with the modifications presented by the mandible, in which 

 the condylar surface is small, and looks directly backwards. 

 There is no ascending ramus, and the coronoid process is quite 

 rudimentary, — all of which conditions are in direct relation to 

 the nature of the food, and absence of the masticatory move- 

 ments. 



The jugal arch in the Sirenia is enormously developed, being 

 composed of the squamosal and the jugal. The former of these 

 is much thickened and presents upon its external face a smooth 

 convex surface. 



In the Manatus, this process of the squamosal rests loosely upon 

 the process of the malar, which, underlying it, extends back as 

 far as the glenoid, having first formed a rim which is both sub- 

 orbital and post-orbital, besides sending a broad plate downwards 

 and backwards, thereby greatly increasing the vertical breadth. 

 The orbital fossa is separated almost completely from the temporal 

 by a bony partition. 



The surface for the muscular attachments, both of the temporal 

 and masseter, are extensive, while the pterygoid plates and groove 

 are relatively enlarged. The vertical curvature of the arch is 

 great, but the horizontal is inconsiderable. The ascending ramus 

 of the mandible is broad, compressed, with rounded angle, and 

 surmounted by an obliquely-placed small convex condyle, much 

 raised above the molar series. The coronary surface is broad, 

 directed forwards, and but slightly elevated above the condyle. 



In the Dugong (Halicore), the jugal arch is much less massive; 

 there is no post-orbital process from the jugal, and consequently 

 no separation of the orbital and temporal fossae by a bony orbit. 

 The coronoid process of the mandible looks backward. 



Although the horizontal curvature of the arch is very slight in 

 both genera of the Sirenia, the temporal fossae are deepened and 

 extended — conditions owing to the walls of the cranium being 

 compressed in a lateral direction, which materially increases the 

 extent of surface for muscular attachment and development. 



In the order Edentata, the jugal arch also offers unusual modi- 

 fications. In the Myrmecophagidse it is very incomplete, being 

 composed of the proximal end of the jugal, articulating with a 

 narrow projecting process of the maxilla, and a very rudimentary 

 fragment of the squamosal. These separate portions, however, 

 do not meet. In fact, they are widely separated. No boundary 

 exists between the orbital and temporal fossae, the latter being 

 comparatively shallow. The glenoid fossa is a shallow cavity 

 running antero-posteriorly, and well adapted to the pointed, 

 backward projecting condyles of the mandible, whose long, 

 straight horizontal rami present neither coronoid process nor 

 angle. In Cycloturus, the mandible is somewhat arched, and 

 presents a well-marked angular process, as well as coronoid sur- 

 face slightly recurved. 



In the Bradypodidse, containing the two species Bradypus and 

 Choloepus. the arch is imperfect, consisting of the jugal, which 

 is narrow at its articulation with the lacrymal and maxilla, but 

 which, widening out into a broad, compressed plate, terminates 

 posteriorly in two processes, the upper pointing backwards and 

 upwards, while the lower looks downwards and backwards. The 

 straight process of the squamosal, although fairly developed, fails 

 to meet either of those of the jugal. There is a post-orbital 

 process of the frontal, which is best marked in Choloepus. The 

 glenoid is shallow and narrow from side to side. The mandible, 

 widest in Choloepus, develops a rounded convex condylar surface, 

 well raised up from the dental series, while the coronoid surface is 

 large and recurved. The rounded angular process projects back- 

 wards to a considerable extent. The symphysis in both species 



is solidified, while in Choloepus it projects forwards into a spout- 

 like process. The temporal surface for muscular attachment is 

 large, as are also the pterygoid plates. 



In the Dasypodidae. the arch is complete, and in its formation 

 the jugal largely enters. This bone extends from the lacrymal 

 and frontal to the process of the squamosal, the anterior third of 

 which it underlies. There is no post-orbital process of the frontal. 

 The glenoid presents a broad, slightly convex, transverse surface. 

 The pterygoids are small. The mandible has a high ascending 

 ramus, the condyle is transverse and high above the alveoli, 

 while the coronoid surface is large and the angle broad and pro- 

 jecting. 



In the Manidse, the jugal arch is incomplete, owing to the ab- 

 sence of the malar, which, if present would occupy almost the 

 exact centre of the arch, — the length of the squamosal process, 

 and that of the maxillary, being nearly equal on either side. The 

 temporal and orbital fossae form one depression in the side of the 

 skull. The rami of the mandible are slender and straight and 

 without teeth, angle, or coronoid process. The condyle is not 

 raised above the level of the remainder of the ramus 



In the Orycteropidae the jugal arch is complete. The horizon- 

 tal curvature is very slight. The post-orbital process is well 

 developed. The mandible rises high posteriorly, with a coronoid 

 slightly recurved, and with an ascending pointed process on the 

 angular edge below the condyle. 



In the Marsupialia, the jugal arch is always complete, and com- 

 posed of the jugal, resting on the maxilla, and squamosal, the 

 first extending from the lacrymal anteriorly to the glenoid fossa 

 posteriorly, of which it forms the external wall. The process of 

 the squamosal passes above the jugal, being united to it by an 

 almost horizontal suture. The horizontal and vertical curvatures 

 of the arch are considerable, and the space for both temporal and 

 masseter muscular insertions is extensive, and the various ridges 

 and crests are extensive, especially in the families of the Dasyuri- 

 dae and Didelphyidae. The post-orbital of the frontal is present 

 as a rule, although in most forms inconsiderably developed. The 

 ascending ramus of the mandible is less elevated than in several 

 of the orders of the Mammalia. The condyle is but little raised 

 above the molar series. The masseteric fossa is extremely pro- 

 jected at its lower external border The mandible has, with one 

 exception, an inverted border to the angle. 



In the Monetremata, the Echidnidae possess an arch in which 

 the squamosal is compressed, and sends forward a slender, straight 

 process to join the corresponding slight, shaft-like process of 

 the jugal. The horizontal curvature of the arch is extremely 

 small. 



In the Ornithorynchidae, the arch is made up of the malar rest- 

 ing upon a process of the maxilla, which, passing straight back- 

 wards, unites with the squamosal process that rises far back on 

 the sides of the cranium. While the mandible of the Echidna 

 has hut the rudiments of the parts which usually enter into its 

 formation, that of the Omithorynchus is more fully developed, In 

 relation to the attachment of the horny teeth. 



LETTERS TO THE EDITOR. 



#•» Correspondents are requested to be as brief as possible. The writer's name 

 is in all cases required as proof of good faith. 



On request in advance, one hundred copies of the number containing his 

 communication will be furnished free to any correspondent. 



The editor will be glad to publish any queries consonant with the character 

 of the journal. 



The Mean Distance of the Earth. 



The interlinear readings to Sir Robert S. Ball's " The Course of 

 an Ice Age " which Miss Hayes gives in Science for April 28 have 

 been read and studied with much grateful appreciation by some 

 readers of that book who find the higher mathematics rather 

 slippery ground to walk on without help. On behalf of a group 

 of such readers, I wish to say a few words on the interlinear read- 

 ing given for the first selection from Sir Robert's book. 



The passage is: "There can be no doubt that when the eccen- 

 tricity is at its highest point the earth is, on the whole, rather 

 nearer the sun, because, while the major axis of the ellipse is un- 



