112 On an Abnormality in Chimsera monstrosa. 



the ventral side, of a median, flat, wedge-shaped body, with 

 a similar but smaller body on each side and ventral to the 

 median one, although the whole of the plate is one (tig. 4). 

 The ligament is rather shorter than in the previous case. The 

 structure of the bone agrees with the dorsal plate of that 

 shown in fig. 2, and I am inclined to regard it as representing 

 that portion only, the ventral portion being absent. 



So far as I am aware, there is no corresponding median 

 bone in this region in any recent fish, the nearest, perhaps, 

 being the jugular plate of Anna, which these certainly do not 

 represent. As remarked, the bone, in the first case, is 

 strikingly like the urohyal of some Teleostei, but, apart from 

 other considerations, its situation in close proximity to the 

 mandibular symphysis at once shows that this is nothing 

 more than an external likeness. 



Jaekel * and Bashford Dean f have both shown that the 

 structure of the jaws in the Holocephali have undergone 

 considerable modification in connection with the development 

 of the paired grinding-plates. On the lower jaw, which 

 consists of two rami fused in front, there are present in 

 CMmcera only two of these plates, but in the Jurassic Chimse- 

 roid, Myriacanthus, in the region of the mandibular symphysis 

 there " occurs an azygous chisel-shaped tooth which is 

 known only in this genus and in the kindred Chimceropsis" ; 

 in both of these instances the tooth is presymphysial. 

 According to Schauinsland J there is no trace of this median 

 unpaired mandibular tooth in the earlier stages of recent 

 (Jhimseroids. 



The question naturally arises, Can these abnormalities in 

 any way be regarded as reversions? The difficulty in doing 

 so is undoubtedly, on first consideration, their postsymphysial 

 position; but if we bear in mind that of the recent forms 

 of Holocephali in Chimcera there has undoubtedly been a 

 shortening of the snout and a more postero-ventral curve of 

 the lower jaw, it is not difficult to imagine a form in which 

 the presymphysial mandibular tooth took up a position similar 

 to that of the abnormality here described. If, however, it 

 does not represent this mandibular tooth, I am unable to 

 offer any suggestion as to its probable homology. 



There is, however, one further fact that lends weight to 

 this probable homology, and that is the nature of the minute 



* N. Jahrb. f. Min. 1901, Beil. 14. 



t " Chiniferoid Fishes and their Development," Carnegie Inst,, Wash- 

 ington, no. 32 (1906). 



\ "Beitrage z. Entwick. und Auatomie der Wirbelthiere," Zoologica, 

 1903, vol, xvi. Heft 39, pp. 1-98, pis. xii.-xxiv. 



