192 Messrs. Hancock & Atthey on Diptenis and Ctenodus, 



formed expansion is much produced, while the anterior angle 

 is only slightly produced. The frontal portion (the pre- 

 sphenoid) is rounded, inclining to conical at the extremity, 

 and fits in between the divergent bones that support the dental 

 plates. The lozenge-formed expansion lies partly behind 

 these bones ; and the elongated posterior extension (the basi- 

 sphenoid) is continued for a considerable distance further 

 back, in the large species for nearly live inches. It is 

 therefore pretty clear that in Dipterus^ in which the sphenoid 

 reaches only a short way behind the extremities of the palato- 

 pterygoids, the head is proportionately short in comparison 

 with that of Ctenodus, in which it must be much elongated. 



We have in our possession numerous sphenoids, belonging 

 to five or six species, three of which demonstrably are those 

 of C. tuherculatus, C. ohliquus, and C. elegans, respectively. 

 They are all very similar in character, varying only a little in 

 the proportions of the parts. The largest are seven or eight 

 inches long ; the smallest, that of C. elegav.s, is only half an 

 inch in length ; the usual size is five or six inches. The basi- 

 sphenoid at its junction with the lozenge-formed expansion is 

 usually thick and nearly circular ; elsewhere it is flattened. 



In Dipterus, too, the vertebrae are ossified ; but there is no- 

 thing to show that this is the case in Ctenochis. Indeed the 

 total absence of any appearance of vertebrae in the specimen 

 of C. elegans before referred to is a pretty good proof that in 

 this genus the central axis of the skeleton was cartilaginous. 



The above distinctive features will perhaps be considered 

 sufficient to warrant the generic separation of these two forms, 

 notwithstanding their evidently close relationship — and this 

 without referring to the minute structure of the dental plates, 

 which exhibits nevertheless some diversity in character. 



In proof of the relationship of the two genera we have only to 

 look to the general form of the oral armature, and to the manner 

 in which the dental plates are placed in the mouth. We have 

 already noticed the similarity of the palato-pterygoid bones to 

 which the upper dental plates are attached, and have pointed 

 out that, while in the one genus the bones are distinct, they 

 are in the other united so that no suture is perceptible. The 

 mandibles (PI. XIII. fig. 1) are also very much alike in both 

 genera, and so is their relationship to the dental plates. 



The cranial bones of Ctenodus, so far as we are able to 

 compare them, also closely resemble those of Dipterus, in 

 which the whole of them appear to have been determined. 

 Unfortunately, only those of the posterior part of the skull 

 are known in Ctenodus. A fragmentary specimen of the 

 occipital region of C. tvherculatus in our possession exhibits 



