230 0. P. Hay — Notes on species of Ichthyodecles. 



Gillicus. 



Characters. A Saurocephalid with maxilla falciform, rela- 

 tively short ; teeth with roots, numerous, small. Gape of mouth 

 smaller than in Ichthyodectes. Mandible deep. Base of skull 

 with strong upward flexure. Scales large. 



The genus is named in honor of Dr. Theodore Gill of the 

 National Museum, who has also made additions to our knowl- 

 edge of the fossil fishes. The generic name is formed in 

 analogy with the word Aristophanicus. 



As type of this genus I take Crook's /. polymicrodus, since 

 it has been figured. Should this be shown to be the same as 

 Cope's L arcuatus, the latter name of course takes precedence. 



Crook has interpreted the bones of the posterior upper 

 region of the skull of his 1. polymicrodus as he has those of 

 Portheus (Xip/iactinus) ; but, as in the case of the latter I am 

 compelled to differ with him. However, many of the sutures 

 are very difficult to demonstrate. According to Crook, the 

 supraoccipital is greatly expanded in front, while the parietals 

 lie laterad of the epiotics. It seems to me that the supposed 

 expanded portion of the supraoccipital is really the area be- 

 longing to the parietals. There is a very distinct fold running 

 along the hinder border of the expansion, and this, continuing 

 up to the midline, has the appearance of a suture. In the 

 crushed specimen before me, there is, along the middle line of 

 the expansion, a break in the bone, but whether due to a frac- 

 ture or to the parting of the bones along a suture, I cannot de- 

 termine. Possibly the parietals were coossified along the mid- 

 line. As to Crook's parietal, I cannot convince myself that 

 there is any suture cutting off the area assigned to it from that 

 assigned to the pterotic. I regard as pterotic the whole area 

 extending from the posterior external angle of the skull to the 

 lateral expansion considered by Crook as supraoccipital. See 

 Crook's fig. 5, pi. xvi, Sq and Fa. 



Crook's determinations of the prootic and the opisthotic are 

 undoubtedly erroneous. His prootic is nothing but the lateral 

 process of the parasphenoid which rises, on each side, to 

 bound the eye-muscle canal. His opisthotic is the true prootic. 

 It rises to the post-frontal, and assists in forming the articular 

 surface for the hyomandibular, just as it does in Xiphactinus 

 and our modern tarpon. The opisthotic is to be looked for 

 further behind, and it seems to me that it occupies the whole 

 or part of the area of Crook's figure 6, pi. xvi, which is desig- 

 nated by Ocl. There is in the side of the skull a deep excava- 

 tion such as existed in the skull of Xiphactinus and exists in 

 the skull of Tarpon • and the specimen in my hands and Dr. 

 Crook's figure 6 give me the impression that, as in Tarpon 



