CONTRIBUTIONS TO PALEONTOLOGY. 299 



he knew anything about the genera into which the group was afterwards sub- 

 divided.* Consequently it is impossible that he could have intended to confine 

 the genus to any one of them, as is now affirmed by some of the naturalists 

 who are opposed to the classification advocated iu this paper. Instead of 

 excluding species with an imperforate beak, such as A. tumida, the etymology 

 of the word Athyris (without a door or opening), the expression ' in which 

 there is no vestige of either foramen, cardinal area, or hinge-line,' and, also, his 

 typical figures all induce the belief that he had before him one or more forms 

 with the beak entire. This is rendered certain by what he says on page 147. 

 Speaking of what he calls A. concentrica^ he says : 'This species is not uncom- 

 mon ; it is figured iu the Bull, de la 8oc. Geol. de France, with a perforated 

 beak, as iu Teeebratula. I have, however, seen numerous specimens with the 

 beak entire and imperforate, as in the other Palseozoic species.' It is highly 

 probable, from all this, that he had in view such Silurian forms as A. tumida. 

 This latter species is so common that it is almost certain that such a collection 

 as he was then engaged upon would contain one or more specimeus." 



This, I suppose, we may consider as " unsophisticated" reasoning in 

 relation to the subject. It amounts to this: " Prof. M'Coy had in his 

 mind the imperforate A. tumida as the type of his genus ; but being 

 engaged in describing Carboniferous fossils, he did not mention it, and 

 followed his generic description with A. concentrica, etc., which species he 

 says he had seen with imperforate beak." 



We think Prof. M'Cot did just what other naturalists would have done 

 under similar circumstances. Seeing the necessity of a separation of 

 these forms from Terebratula, he proposed the obvious characters on 

 which the distinction was founded. The question of perforate and imper- 

 forate beaks in this group of fossils may be open to discussion ; at least 

 in all that I have examined I have found no entire beaks ; but in some 

 of the gibbous forms the apex is so incurved as to give an ' ' apparently 

 imperlbrate " character, and the species are thus cited. In some species 

 this perforation is well defined by deltidial plates below ; but generally 

 these are absent, and the upper side of the foramen presents a semi- 

 circular outline, communicating with a triangular space which at some 

 period has been occupied by the deltidial plates. After a careful exam- 

 ination of hundreds of specimens, I am compelled to conclude that this 

 feature, or its modification, is not reliable for specific distinctions, and 

 certainly not of generic importance. 



Mr. Billings, after citing the list of species placed under the Genus 

 Spirigbra by M. D'Orbigny, remarks as follows : 



* Is it necessary that an author should know what is afterwards to he discovered, in order to 

 understand what he intends to do at the present time ? 



