Genera Palseclaplms and Holodus. 13 



By Messrs. Van Beneden and De Koninck this fossil was 

 considered to be the anterior part of the head of a fish which 

 belonged " sans contestation a I'ordre des Plagiostomes et au 

 sous-ordre des Squalidiens." The two dental plates they re- 

 garded as the two halves of the upper jaw, the anterior rounded 

 ridge as the upper lip ; and, lastly, as regards the two lateral 

 fossae or excavations (?/), they considered that they " ont pro- 

 bablement correspondu aux fosses nasales de I'animal." No 

 traceof eyes could, however, be discovered; and, in conclusion, 

 they held that this curious fish approached on the one hand 

 the Chimffiroids, on the other the Cestracionts and Squatinids. 



Some time afterwards another and apparently closely allied 

 fossil was described by Prof. Van Beneden from the Devonian 

 strata of Belgium *. This consists of a large dental plate 

 closely united along nearly its entire length with its fellow of 

 the opposite side, and presenting fine ridges, anteriorly tuber- 

 culated or crenulated, and slightly diverging from each other 

 as they pass from behind forwards. To this specimen Prof. 

 Van Beneden gave the name of Palcedaphus devoniensiSj con- 

 sidering it to be the upper jaw of a Plagiostome of the same 

 genus as that previously described by himself and Prof. 

 De Koninck. 



Pander had, however, already, in 1858, described, under the 

 name oi Holodus "f, a fragment from the Devonian of Bussia, 

 which, though of much smaller size, bears a most unmistakable 

 resemblance to PaJa^dajjhiis insignis, being, in fact, evidently 

 the very same part of the head of a closely allied animal. On 

 comparing Pander's figure of Holodus with those of Palceda- 

 phus insignis it will be seen that, like the latter, the fragment 

 consists also of two united and diverging rami, broken off 

 close behind their union. On one aspect we have a smooth 

 ganoid surface undivided by any median suture, and reflected 

 over in front so as to form a lip-like margin ; on the opposite 

 aspect are two dental plates whose outer mai-gins are elevated 

 and bear each at least two conical tooth-like tubercles ; these 

 dental plates do not, however, as in Palwdaijlms, touch each 

 other in the middle line, though the bones on which they are 

 carried are united by median symphysis. Seen from the side, 

 the lip-like margin is truncated as in P. insignis, and shows 

 here exactly the same right and left depressions or hollows, 

 supposed in the Belgian fossil to be nasal fossa3. 



This fossil [Holodus) was also considered by Pander to be 

 the snout of a fish ; the bones bearing the tooth-plates are 

 compared by him to the palato-pterygoids oiDipterus', and into 

 the angle where they diverge posteriorly he conceives an " os 



* BuU. Ac. Belg. (2) xxvii. 1869, pp. 37.8-385. 

 t ' Ctenodipterinen/ pp. 38-43. 



