183 

 ISOSPONDYLI. 



saurodontim;. 



This family embraces carnivorous fishes, many of them of large size 

 and interesting structure, which have as yet only been discovered in the beds 

 of the Cretaceous formation. They are of interest to the student of com- 

 parative anatomy, and also to the paleontologist, as they appear to have been 

 the predominant type of marine fishes during the Cretaceous period in the 

 North American seas, and to have been abundant in those of Europe. 



The characters already assigned to the family are confirmed by the new 

 species discovered, and many additional ones added, as follows : 



The cranial structure can be nearly made out, and the following points 

 may be regarded as ascertained. The brain-case is not continued between 

 the orbits, and the basis cranii is double and with the muscular tube open. 

 There are no exoccipital condyles, and that of the basioccipital is a conic 

 cup. The homologies of some of the bones that constitute the cranial walls 

 are difficult to determine. The basioccipital is longitudinally excavated 

 below. The exoccipital is probably a small bone, which embraces the basi- 

 occipital closely, so that it is difficult to say whether the bone that joins the 

 opisthotic below is the former or latter. The opisthotic has considerable 

 transverse extent, and an articular surface behind, probably for the posttem- 

 poral. The supraoccipital is keeled or longitudinally crested above, and is 

 preceded by, or continued into, a longitudinal median extension, which contin- 

 ues as far as the frontal bones on the middle line, separating entirely the 

 lateral elements. It is uncertain whether this be supraoccipital, or, by 

 homology with the Siluroids, conjoined parietal bones. If this homology be 

 true, we can easily refer the elements which bound the exoccipitals above, 

 and the supraoccipitals on either side, to the epiotics. They occupy the posi- 

 tion of the epiotics in Salmo, and are produced upward and backward into 

 crests which have free margins, both on the upper and postero-interior mar- 

 gins. The appearance of these formerly led me to suspect the presence of a 

 fontanelle, which I am now able to assert has no existence. The extent of 

 these supposed epiotics anteriorly is limited by the approach of the more lateral 

 elements to the middle line. These elements are wide, and, offering attach- 

 ment to the opisthotic, hyomandibular, and postfrontal, must be regarded as 

 pterotic. The postfrontal is a well-developed bone. The frontals terminate 



