1884.] 43 [Cope. 



Should Albrecht's determination of the homology of the quadrate bone 

 prove to be correct, the quadratojugal arch is the zygomatic, and the 

 superior arch becomes the accessory one. This being admitted, the Lacer- 

 tilia cannot be said to have a zygomatic arch, and the Theromorpha do not 

 possess their postorbito-squamosal arch ; the diversity between the two 

 orders being thus greater than has been supposed. 



The articulation op the ribs in Embolophorus. — The ribs of the 

 Theromorpha are two-headed. While the tubercular articulation has the 

 usual position at the extremity of the diapophysis, the capitular is not 

 distinctly, or is but partially indicated, on the anterior edge of the cen- 

 trum, in Clepsydrops and Dimetrodon. In Embolophorus, as I showed in 

 1869, the capitular articulation is distinctly to the intercentrum. A second 

 and larger species of that genus, recently come to hand, displays this 

 character in a striking degree, since the intercentrum possesses on each 

 side a short process with a concave articular facet for the head of the ribs. 

 From the slight corresponding contact with the intercentrum seen in 

 Dimetrodon and other genera, there can be little doubt that this is the 

 true homology of the ribs in the order Theromorpha. 



The consequence follows from this determination, that the ribs of this 

 order are intercentral and not central elements, and that they do not there- 

 fore belong to the true vertebrae, thus agreeing with the chevron bones, 

 with which they are homologous. 



It is also true that this type of rib-articulation approximates closely that 

 of the Mammalia, where the capitular articulation is in a fossa excavated 

 from two adjacent vertebrae. This is what would result if the intercen- 

 trum were removed from a Theromorph reptile, and the head of the rib 

 allowed to rest in the fissure between the centra left by the removal. It is 

 well known, that the double rib articulation of the other reptilian orders 

 which possess it, viz.: Ichthyopterygia, Crocodilia, Dinosauria and Ptero- 

 sauria, and in the birds, is different, the capitular connection being below 

 the tubercular, on the centrum. Whether the capitular articulations and 

 the ribs in these orders are homologous with those of the Theromorpha, 

 remains to be ascertained, 



The origin op the Mammalia. — The relation of the characters of the 

 Pelycosaurian suborder of the Theromorpha to those of the Mammalia 

 may now be seen to be very important. I give a synopsis of the charac" 

 ters of these divisions parallel with those of the Batrachia contemporary 

 with them, in order to give a clear idea of the reasons for believing that 

 the Mammalia are the descendants of the Pelycosauria. 



The following table shows that the Mammalia agree with the Batrachia 

 in two and part of another character ; with the Pelycosauria in six char- 

 acters, and with other Reptilia in two characters. The Pelycosauria agree 

 with the Batrachia in two and in parts of two other characters, and with 

 other Reptilia in three characters, two of which (Nos. 2 and 3) are of 

 prime importance. Of the characters in which the Pelycosauria agree 



