210 DE. W. G. EIDEWOOD ON THE CEANIAL OSTEOLOGY OE 



however, instead of running horizontally, have an upward tilt, 

 and are directed towards the summit of the first pharyngo- 

 branchial, a long, nearly vertical, rod-like bone, which here 

 simulates a spicular bone and is attached to the side of the 

 pro-otic. The fourth epibranchial is an expanded plate of bone, 

 roughly triangular in shape. 



Summary of the GJiaracters of the Skull in the Families 

 Mormyridse, Notopteridse, and Hyodontidse. 



The foregoing observations may be summarized as follows : — 

 In Notopterus, Syodon, and the Mormyridae the parietals meet 

 in the median line ; teeth occur on the parasphenoid (although 

 they are rudimentary in Mormyrops and absent in Gymnarchus) ; 

 an orbitosphenoid is present, and the cavum cranii of the dried 

 sknll is continued through this bone to the ethmoid region. 

 An opisthotic is present in Notopterus ; it is exceptionally large 

 in Syodon ; it is absent in the Mormyridse. A basisphenoid 

 is present in Notopterus and Hyodon, and in Petrocephalus and 

 Gymnarchus, but not in the other Mormyridse examined. The 

 eye-muscle canal opens posteriorly by an oval foramen in 

 Syodon, but is blind posteriorly in Notopterus and the 

 Mormyridse. 



The postfrontal is small in Notopterus, and does not form 

 part of the articular facet for the head of the hyomandibular, 

 whereas in Syodon and the Mormyridse it is larger and has 

 the more normal relation ; it is small, however, in Gymnarchus. 

 A pair of diverticula of the swim-bladder, of fair size, occur on 

 the sides of the basioccipital in Notopterus and Syodon ; their 

 ventral and external walls are of strong fibrous tissue, but 

 cranial bones form the dorsal and internal walls. In the 

 Mormyridse the basicranial bones are normal. 



In Notopterus and the Mormyridsa there is a large lateral 

 cranial foramen bounded by the epiotic, squamosal, and ex- 

 occipital. This, in the Mormyridse, is loosely covered over by 

 the large, thin, scale-like supratemporal, but in Notopterus the 

 supratemporal is so small that it fails to form a cover for the 

 foramen. The lateral foramen is not present in Syodon, but 

 there is a cartilaginous tract, bounded by the parietal, epiotic, 

 and squamosal. The supratemporal of Syodon is large and 

 scale-like, but it is firmer than that of the Mormyridse ; the 



