SUPPLEMENT TO DECADE VIII. 



3 



only simulates the tail of a true heterocerque fish, but carries the resem- 

 blance to an extent onlj found in the most heterocerque genera. I 

 question whether any one on seeing a drawing of this fin would hesitate a 

 moment in pronouncing it a palseozoic form. The solution of this pro- 

 blem depends upon a single point, viz., whether the rays constituting 

 the upper lobe of the tail are all short rays, given off from the lower 

 elements of the vertebral column, or whether any of them are continued 

 under the scaly integument to the upper part of the column. The evidence 

 the specimen affords is this : the centrum in the vertebrae of this genus 

 was ossified, a fact proved by the occurrence of several of these bodies, 

 seen in the specimen, where the integuments have been removed. Being 

 thus qualified to resist decomposition, while the softer parts perished, the 

 course of the spinal column becomes evident by a slight elevation of the 

 surface where the scaly integument rests upon it. In tracing its direction 

 in the posterior part of this fish, it exhibits no tendency to mount into the 

 upper lobe of the tail, as in the typical heterocerque fish, and, to a 

 certain extent, in Ophiopsis, Eiignathus, and some other homocerque forms; 

 but, on the contrary, it seems to terminate abruptly at the base of the 

 tail. For this reason I am inclined to think that, without more conclusive 

 evidence, it would be unwise to consider this an exception to the general 

 rule, with reference to the fish of the lias, although the actual resemblance 

 of this tail to that of a heterocerque is so striking as almost to warrant a 

 contrary conclusion. 



Article XII. Plate X. 



Pycnodus Uassicus.— The opinion I ventured to express in the descrip- 

 tion of this specimen, that the more shortened contour of the body as 

 compared with the typical Pt/cnodus platessus ought not to exclude it 

 from that genus, has been fully confirmed by the discovery of several 

 new species of Pycnodus in the oolitic slates of the Bugey. Monsieur 

 Thiolliere, in one of the most splendid works ever contributed to palaeon- 

 tological literature,* has described and figured, together with other new 

 and interesting forms, five species of this genus. Of these two are more 

 elongated than Pycnodus platessus, and of the three shorter species two, 

 viz., Pycnodus Egertoni and Pycnodus Pernardi, correspond in form witli 

 liassic species described in this Decade. There seems to be some doubt as 

 to the locality from which the British Museum example of this fish was 

 derived. In the course of last autumn I saw a second specimen of this 

 species presented to the Worcester Museum by the Rev. W. Symonds, 

 which was found in the lias of the neighbourhood of Tewkesbury. 



P. DE M. Grey Egerton. 



April 16, 1855. 



* Descriptions des Poissons Eossiles provenont des Gisements corallines clu Jura, dans 

 le Bugey. 



