BRITISH FOSSILS. 



.3 



traces of them when viewed from within ; they are often distinct 

 (Hall, 1. c. 231). Burmeister also calls attention to this character. It 

 appears to be frequent in the genus and its allies, for I have seen 

 Swedish specimens, both of Asajphus and Illmnus, which exhibited 

 it strongly, chiefly on the axis. In like manner the lobes of the 

 glabella often show internally, though quite obliterated on the ex- 

 ternal crust. I have not seen them in this species, but Burmeister 

 gives them in his figure, of which he boasts the absolute accuracy. 

 We can at least say as much for ours, so far as the specimens exhibit 

 character ; and our plate of this species is alike creditable to the 

 artist, Mr. Bone, and the engraver, Mr. Lowry. 



Variations. — There seems to be some reason to think there may 

 be two species in the American limestones ; one rarer, of broad form 

 and with small eyes— the true A. platycephalus of Stokes ; the 

 other very common, of elongate form, and with variably large eyes, 

 to which nearly all the above synonyms belong. See figures of 

 both forms in the " Geology of Canada," 1863, by Sir W. Logan and 

 E. BiUings, p. 184. I have quoted these above. On the other hand, 

 I should have no difiiculty in referring these differences to sex, the 

 hrosid A, platycephalus being the $ form. 



History. — It is not necessary to refer to the American authors, who 

 have profusely illustrated this common species. But I feel persuaded 

 that Prof Hall is right in uniting all Green's casts under one name ; 

 the more so as Hall had several hundred specimens at command in 

 every degree of perfection. Prof. Green's species are chiefiy due to 

 differences of position, and in attempting too closely to identify 

 each of these, Gen. Portlock separated the Irish specimens he de- 

 scribed into more forms than can be now admitted. I. gigas and 

 /. planus of Portlock he himself considers identical. I. ovatus, id., 

 difi'ers in nothing but its size ; the head is really not more elongate 

 than in his specimens of /. planus; and with regard to his 

 I. Powisii, the more depressed form is entirely due to pressure ; 

 and the fulcrum is at the same proportional distance, — about one- 

 third from the axis (not more distant, as stated) ; the appearance 

 is due only to the before-mentioned cause. The true A. Poiuisii, 

 Murch., has distinct ribs to the tail, square ends to the pleurae, and 

 a swelled glabella, but the Irish specimens all resemple I. gigas in 

 these respects. I do not know that the true A. Poivisii occurs 

 in Ireland, while /. gigas is not known in England. 



The large Isotelus megistos, which is certainly the same species 

 with I. gigas, has been reconstructed in the form of a cast by 



11 c 2 



