GREGORY: FISH SKULLS 



151 



tions will be more fully recorded in later papers but for the purposes of the present report 

 the following comments may be made. 



Opisihoproctus. — This deep-sea fish (Fig. 43) is strongly reminiscent of the Cretaceous 

 Ctenothrissa microcephala, as figured by Smith Woodward (1901, PI. X). To put it the 

 other way around, the ancient genus approaches the-modern in its short deep body, broad 

 caudal peduncle, very large thin cycloid scales, very short deep head with very large orbit, 

 as well as in its forwardly-produced suspensorium, small narrow upturned mouth with 

 very high ascending process of dentary; the preopercular likewise has a boomerang-like 

 elbow and the opercular is deep. 



On the other hand, the Cretaceous fish differs from its modern analogue in retaining 

 such primitive characters as two supramaxillae and in the more normal form of the orbits. 

 A noteworthy difference is the anterior position of the pelvic fins, which lie beneath the 

 cleithra in Ctenothrissa but are abdominal in Opisihoproctus. Ctenothrissa also lacks the 

 flattened abdomen and the pocket lying between the lower border of the mandible and the 

 anteriorly prolonged branches of the cleithra, which is one of the most peculiar features 

 of Opisihoproctus. These are obviously specializations, perhaps of relatively late date. 

 An alternate possibility is that Opisihoproctus, which retains the adipose dorsal fin that is 

 so characteristic of the Salmonidse and their allies, may be derived from some small 

 Argentina-\i\it form having very large eyes and a small mouth set at the end of a slightly 

 elongate oropharyngeal tunnel. 



Chanos. — The peculiar genus Chanos (Fig. 44) differs from the Clupeidae in many 



pet, soc 



vo 



'^^^ dsphot 

 fr ' ^ 



'Sbtm' 

 • scakbone 



lop- 



Chanos salmoneus 



^iks 



Fig. 44. Chanos salmoneus. After Ridewood. 



