8 



DIADECTID^E. 



I have obtained three skulls of the Empedocles molaris, a species of this 

 family, which display the occiput, and two of them the basis of the cran- 

 ial and facial regions. From them I derive the following characters.* 



The relations of the quadrate and zygomatic arches are as in the Thero- 

 morpha generally. The pterygoids extend to the quadrates, and the vomer 

 bears teeth. The brain-case extends to between the orbits, and its lateral 

 walls are uninterrupted by fissures from this point to near the origin of the 

 os quadratum. There is an enormous frontoparietal foramen. The mode of 

 connection with the atlas is peculiar. There is a plane facet on each side of 

 the foramen magnum, which then expands largely below them. The bone 

 which bounds it inferiorly, presents on its posterior edge a median concavity. 

 On each side of this, is a transverse cotylus, much like those of an atlas which 

 are applied to the occipital condyles of the Mammalia. They occupy pre- 

 cisely the position of the Mammalian condyles. The median point of their 

 upper border, which forms the floor of the foramen magnum, is produced 

 in the position occupied by the median occipital condyle of a reptile. 

 From its position between the cotyli, the section of this process is triangular. 

 The element in which the cotyli are excavated has the form of the mam- 

 malian basioccipital, and of the reptilian sphenoid. It is not the batrachian 

 parasphenoid. Its extreme external border on each side where it joins a 

 crest descending from the exoccipital, is excavated by a circular fossa 

 which looks outwards. 



The character of this articulation is so distinct from anything yet known 

 among vertebrated animals, that I felt justified in proposing (1. c, p. 304) 

 a new division of the Theromorpha to include the Diadeetidm, to be called 

 the Cotylosauvia. The superior facets described, indicate the presence of 

 atlantal zygapophyses as in the Ganocephala. 



There are three genera of Diadeetidm, one of which is now introduced 

 for the first time. • They are distinguished as follows : 

 I. Molar teeth in one series ; 



A distinct canine Diadeetes. 



No canine Empedocles. 



II. Molar teeth in two series ; 



A canine Helodectes. 



I am acquainted with six species of this family, two of each of the genera. 



DIADECTES Cope. 



Proceeds. Amer. Philos. Society, 1878, p. 505. American Naturalist, 

 April 22, 1878. 



The typical species ot the genus has compressed teeth, with one end of 

 the crown much more elevated than the other. In the lower jaw the inner 

 extremity is the elevated one, and Wee versa. There is a large tooth in the 

 position of a canine in the inferior'series, but it is not certain whether or 

 not it is an incisor. A new species is now described which is intermediate 



*These were first described in the American Naturalist, 1880, p. 304. 



