COPE ON OWEN ON PYTHONOMOKPHA. 303 



langes and of digits in the limbs of PythonomorpJia remains to be ascer- 

 tained, I reply that this part of the structure of these reptiles has been 

 made known by Professor Marsh.* 



IT. — HOMOLOGICAL DETERMINATIONS. 



The determinations of this kind which I have made are, with two ex- 

 ceptions, confirmed by Professor Owen. Among them is one to which 

 I attached some importance in the definition of the Ft/tJionomorpha, viz: 

 the identification of the opisthotic bone of that order with the suspen- 

 sorium of the snakes, in opposition to the view of Huxley, that the latter 

 bone is the squamosal. 



The two exceptions are important. The one is the determination of 

 the pterygoid bone; the other that of the roots of the teeth. 



The arch which connects the solid palate with the distal portion of 

 the OS quadratum in the Pythonomorpha includes two bones, an anterior 

 dentigerous one, and a i)osterior edentulous one. The posterior bone is 

 not described by either Cuvier or Owen, and was probably unknown to 

 them; hence, believing that the anterior bone is the posterior one, they 

 termed it the pterygoid, and gave the name palatine to the horizontal 

 elements immediately in front of the latter, and which I have heretofore 

 regarded as its anterior portion. The latter (No. 20 of Professor Owen's 

 fig. 16, Quar. Journ. Geol. Soc. 1877, p. 695) is, however, regarded by 

 Owen as a distinct element, and he finds an oblique suture separating 

 it from the dentigerous posterior region. On examination of the speci- 

 men of Clidastes propytJion, I find that there is a squamosal suture in the 

 I)Osition indicated by Owen, so that it is now evident that the posterior 

 dentigerous element is the true pterygoid, as determined by Cuvier. 

 The posterior portion of this bone is deeply excavated, and the portion 

 which diverges outward and backward from this point I have regarded 

 as a distinct element. It is bounded anteriorly by a groove, which 

 nearly resembles a suture. This groove is not continued on the inner 

 side, so that it is either a groove for muscular insertion or a trace of a 

 suture now obliterated. So it cannot be maintained that this posterior 

 portion of the pterygoid is a distinct element. In this point I am cor- 

 rected by Professor Owen. 



As regards the teeth of the PytJwnomorpJia, I have stated that they 

 "possess no true roots". Upon this Professor Owen responds categori- 

 cally, — "The teeth of Mosasauroids have an enamelled crown and cement- 

 clad roots." To this I must reply that my statement is in accordance 

 with the fact and with the views of Cuvier and Leidy. The crown of 

 the tooth in this order is supported on an osseous pedicel, which is 

 not a true root, *. e., it contains no dental tissue. By reference to Dr. 

 Leidy 's figure f and description of a section of a tooth of Mosasaurus, it 



*Amer. Journ. Sci. Arts., June, 1872, pi. x. 



t Cretaceous Reptiles of North America, pi. xx, f. 3, p, 50. 



