1805] 63 [Slade. 



sive, and that of the pterygoid considerable. The ascending ramus of the 

 mandible is high, and the condyle small and round. The coronoid pro- 

 cess is compressed, and but little elevated above the molar series, while 

 the angle is thickened and rounded posteriorly. 



The jugal arch in the order of the Cetacea presents some singular modi- 

 fications. In the Delphinoidea, the squamosal, frontal and jugal enter 

 into its composition. The squamosal sends forward a large, bulky process 

 which nearly meets the descending postorbital process of the frontal. 

 The jugal is an irregular flat bone, covered by the maxilla, and sends 

 back from its anterior and internal border a long and very slender pro- 

 cess, curved slightly downwards, to articulate with the short obtuse pro- 

 cess of the squamosal, thereby forming the lower boundary of the orbit. 

 So far as the relations of the squamosal and frontal are concerned, the 

 portion of the arch thus formed is a counterpart of that of the horse : 

 although the union of the two bones is much more complete in the latter 

 animal. The jugal in the horse is relatively a much larger bone, and 

 sends back a well-developed process jvhich underlies that of the squamo- 

 sal, with which it is joined by a nearly horizontal suture, thus forming a 

 strong suborbital bony wall. The delicate character of the suborbital 

 process of the jugal, and its union with the squamosal in the Delphinoidea, 

 render it difficult at first sight to determine its relation to the arch, and 

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

 not be disputed. 



In the Balfenoidea, 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 correlated 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 pro- 

 cess is quite rudimentary — all of which conditions are in direct relation 

 to the nature of the food, and absence of the masticatory movements. 



In the Sirenia, the arch is greatly 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 whi-ch is both suborbital and postorbital, 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 attach- 

 ments, both of the temporal and masseter, are extensive, while the ptery- 

 goid 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 sur- 

 mounted 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. 



