Cope.] 4 J [August 15, 



is unnecessary to observe however, that this part of the skeleton does not 

 resemble the corresponding part in the known Monotremes. 



Structure of the quadrate bone in the genus Clepsydrops. — 

 The quadrate bone in Clepsydrops leptocephalus Cope, already described, is 

 of highly interesting form. It consists of two portions, a vertical and a 

 transverse, the latter much longer than the former. The vertical portion 

 is wedge-shaped with the base fashioned into the condyle for the mandib- 

 ular ramus. Its posterior face to the apex, is articulated with the large 

 squamosal, which rises towards the parietal bone. The distal part of the 

 quadrate is grooved anteriorly, and each edge sends a process forwards. 

 The internal is short, and articulates with the pterygoid. The external is 

 the long horizontal part of the bone already mentioned. It is compressed, 

 and at the end is acuminate. Although the malar bone is out of place in 

 the specimen described, examination of the skull of the Clepsydrops natalis, 

 where it is preserved in position, shows that this horizontal ramus of the 

 quadrate is nothing more than the zygomatic process of the squamosal 

 bone of the Mammalia, forming with the malar bone the zygomatic arch. 

 In the Pelycosauria there is but one posterior lateral arch, as is demonstrated 

 by many specimens ; hence, we have here a reptile with a zygomatic arch 

 attached to the distal extremity of the quadrate bone. 



Important results follow this determination. We have seen that, with 

 Peters, we need no longer look to the auricular chain of ossicles, and 

 especially to the incus, to find the homologue of the os quadratum of the 

 Vertebrata below the Mammalia. According to Albrecht the os quadratum 

 is the homologue of the zygomatic portion of the squamosal bone. If this 

 be true, in the process of specialization of the reptiles, the anterior or zygo- 

 matic portion of the quadrate has been lost or separated as a quadrato- 

 jugal bone, and the condylar portion extended, until it has reached the 

 extreme length we observe in snakes. This determination of the character 

 of the quadrate bone in the Theromorphous Reptilia is confirmatory of 

 the theory broached by Albrecht.* Among many propositions novel 

 to the science of osteology, none has been more unexpected than 

 his assertion that the quadrate bone is the homologue of the zygo- 

 matic and glenoid portion of the squamosal bone of Mammalia. This is 

 in contradiction to the view held by many comparative anatomists from 

 the day of Reichert to the present time. 



I made a study of these arches several years ago, which is published in 

 the Proceedings of the American Association Adv. Science, Vol. xix, p. 

 18. Accepting the prevailing view that the quadrate bone is one of the 

 auditory ossicles, I naturally homologized the superior arch of the rep- 

 tilian skull, which articulates with the squamosal proper, with the zygo- 

 matic arch, and looked upon the quadratojugal arch as an additional 

 structure, connected with the peculiar development of the supposed incus. 



* Sur la valeur morphologique de l'articulatlon maudibulaire et des osselets 

 de l'oreille, etc , Bruxelles, Mayolez, 1883. 



