212 DR. W. G. EIDEWOOD ON THE CKANIAL OSTEOLOGY OF 



opercular is also small, tapers to a point posteriorly, and, except 

 in Gymnarchus, is concealed by the opercular ; in Notopterus 

 the subopercular is wanting. 



Considerable differences are to be noticed in the hyobranchial 

 skeleton. A single small hypohyal, probably the upper of the 

 two normally present, is to be found in Notopterus, and the 

 same holds true for Petroceplialus. but in the other Mormyridse 

 examined there is no trace of a hypohyal. In Syodon the upper 

 and lower hypohyals are both present ; they are large and 

 approximately of the same size. 



In Notopterus the first basibranchial is unossified, which is 

 not the case in Hyodon and the Mormyridae. In the Mormyridse 

 there is a lai'ge bone which either represents the first basi- 

 branchial alone or the first basibranchial and the endosteal 

 glossohyal combined; beneath this the urohyal is immovably 

 fixed or is fused. The urohyal in Notopterus and Hyodon, 

 however, is freely movable, and has the usual paired ligament 

 in front ; in Hyodon the bone is quite small. 



In Hyodon the hypobranchials are normal ; in Notopterus, 

 also, they are normal, but there are in addition a pair of tendon- 

 bones projecting downward and outward from the posterior end 

 of the second basibranchial ; in the Mormyridse the first hypo- 

 branchials are extremely short, and the second hypobranchials 

 small and confluent with, the equivalents of the paired tendon- 

 bones of Notopterus. These tendon-bones are not represented 

 in Hyodon. 



The first pharyngobranchial of Notopterus is cartilaginous ; 

 it is also unossified in PetrocepJialus and GymnarcTius, but in 

 Mormyrus and Morniyrops it is ossified, though small ; in Hyodon 

 it has the form of a long, upright rod of bone, simulating a 

 spicular bone. 



Comments on the Skull of the 

 Mormyridse, ISotopteridse, and Hyodontidse. 



On the whole, the study of the craniological characters impels 

 one to the conclusion that the families Mormyridse, Notopteridse, 

 and Hyodontidse, though more closely related inter se than is 

 either family with any other family of Malacopterygian fishes, 

 are not more intimately related with one another than was 

 previously assumed to be the case. A.s has been seen from the 

 summary immediately preceding, the cranial characters of the 



