MORPHOLOGICAL REVISION 



107 



the articular continues the curve of the inner border of the mandible to the outer 

 side of the extreme posterior end of the angular process. The suture between the 

 angular and the surangular passes forward nearly midway of the mandible, and 

 nearly parallel with the upper border in the closed jaws, to the hind end of the den- 

 tary, that is to nearly opposite the posterior end of the orbits. The thin ascending 

 plate of the surangular reaches at the summit nearly as high as the lower margin of 

 the orbits on the inner side of the temporal roof. Over the summit the slender 

 posterior end of the coronoid is visible in the closed jaws, but its anterior part is 

 concealed by the transverse bones. The articular is a short bone, turned inward, 

 with a thin inner margin. It is apparently continued forward as a slender prolonga- 

 tion above the margin of the splenial or angular and forming the lower border of the 

 mandibular cavity, to a slender, acute point nearly as far as the hind end of the 

 mandibular tooth series. Whether or not it is separated from the articular as a 

 distinct bone, the prearticular, or indeed of its precise relations I will not be sure. 

 Sixteen teeth I count in the mandibular series in three different skulls. They resem- 

 ble the maxillary teeth, but are somewhat smaller. The first or second is distinctly 

 larger than the following ones. 



"Perhaps nothing is more noticeable in the skull of the present reptile than the 

 small comparative size of the brain cavity. While the skull measures over seven 

 inches in length and five in width, the foramen magnum is of almost precisely the 

 same size and shape as that of a small Amblyrhynchus lizard whose skull measures 

 but sixty millimeters in length. Not only is the foramen of the same size, but the 

 brain cavity also is only a trifle larger in Lahidosaurus. Small brain capacity is of 

 course to be expected in this old reptile, and, moreover, the size of the brain cavity 

 as compared with that of the skull may not be a fair criterion of the relative intelli- 

 gence of the two animals. Nevertheless, that their intelligence was relatively 

 much lower than that of the existing lizards can not be doubted." 



As shown in the discussion of the skull of Captorhinus, page 96, the bones 

 described by Williston as epiotics can not be such, for these .bones are present as 

 small, triangular nodules between the parietals, squamosals, and the supraoccipital 

 plates (Williston's postparietals). The outer of the two plates on the posterior 

 surface of the skull are, as Williston suggests, probably homologous with the bones 

 of similar position in Dimetrodon and are most probably the quadratojugals; rea- 

 sons have been given for not accepting Williston's objections to this homology as 

 decisive. 



The vertebral column: The specimen described by Case (II) had but eighteen 

 presacral vertebrae; Broili (T) described twenty-two in his specimen, and Williston 

 is of the opinion that at least two must be missing from the specimen described by 

 Case. A complete specimen. No. 4876 in the American Museum, has twenty-five. 

 It is altogether probable that more than one species is represented in the numerous 

 specimens, but it is impossible to determine this point. It is only certain that the 

 genus Lahidosaurus had a maximum number of twenty-five presacral vertebrae. 



The atlas and axis are present in No. 4876 Am. Mus., but are obscured in part 

 by the skull. There is a good-sized atlas centrum, but the neural arches are lost. 

 Broili describes a keel in the lower surface of the centrum in his specimen. Evi- 

 dence of a preatlantal intercentrum is also present. The spine of the axis is broken 

 off and lost, but the neural arch shows it to have been quite heavy. The transverse 

 processes are set high up on the sides of the neural arch and the face for the rib is 

 short; there seems to be a small diapophysis present on the centrum, indicating that 

 the rib was perhaps double-headed. The transverse process of the third vertebra 

 has an elongate face for the wide proximal end of the single-headed rib. 

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