BRITISH FOSSILS. 



Decade YIII. Plate III. 



ASTERACANTHUS SEMI VERRUCOSUS. 



[Genus ASTER ACANTHUS. Agassiz. (Sub-kingdom Yertebrata. Class Pisces 

 Order Placoidei. Family Cestraciontidse.) Dorsal spine large, tuberculate, with a double 

 row of processes on the p sterior margin ; base smooth.] 



Aster acanthus semiverrucosus, Sp. Nov. 



Description. — This Ichthyodorulite is very distinct from the 

 specimen described in the preceding article, so mnch so, that I have 

 no hesitation in considering it a new species. Experience has 

 taught us that in those Placoid fishes which had more than on 

 spine, the second dorsal defence differed more or less in form and 

 other peculiarities from the principal one, and thus specimens con- 

 sidered originally as distinct are now recognized as belonging to 

 one and the same species. For instance, Hyhodus curtus, and 

 H. reticulatus, formerly described in the " Poissons Fossiles as 

 two species, are now shown to have been the first and second dorsal 

 defences of the same fish, and are consequently united under the 

 latter denomination. But there are limits to these variations ; and 

 there is, moreover, a general similarity of character, which the ex- 

 perienced eye cannot fail to detect, although to a certain extent 

 disguised by the deviations from a recognized type. In the subject 

 before us there are the strongest evidences of specific difference from 

 Asteracanthus verrucosus, as will be seen in the sequel. The speci- 

 men is deficient at the point and at the base. The portion remaining 

 measures 7^ inches. If entire, the length would probably have 

 been about 9 inches. The greatest breadth is l-p'o- inch, from which 

 point the spine contracts very gradually. It is more falcate than any 

 other species of the ^ genus. The anterior face is characterised by a 

 strong carina, which |is broken up into tubercles, near the base. 

 The sides of the spine are covered for one half of the entire length 

 with large coarse tubercles, irregularly arranged, and varying both 

 n shape and size. The largest are near the front, these are the 

 [viii. iii.] 8 D 



