236 Mr. E. Billings on the Genus Athyris. 



dorsal [ventral] valve. 2. Martinia, M'Coy, or the smooth Spirifers, 

 in which the hinge-line is less than the width of the shell, and the 

 cardinal area triangular. 3. Athyris, INI'Coy, in which there is no 

 vestige of either foramen, cardinal area, or liinge-line. This remark- 

 able genus is frequently confounded with those shells usually named 

 Terehratula in the older rocks, but is distinguished by the large 

 spiral appendages, which are wanting in the other group. 4. Ura- 

 chythyris, M'Coy, iu which we find the longitudinally ribbed sur- 

 face of Spirifera united with the short hinge-line of Martinia. 

 f). Orthis, Dal., in which there are no spiral appendages, the hinge- 

 line and striae frequently spinose (as in Lepfcena), the cardinal area 

 common to both valves, and its sides inclined towards each other 

 at its angles ; dorsal valve smallest." — Op. cit. p. 128. 



On page 146 of the same work he thus concisely describes 

 the genus : — 



" Gen. Char. — Nearly orbicular, small ; no cardinal area or hinge- 

 line ; spiral appendages very large, filling the greater part of the 

 shell. 



"This very interesting group possesses all the external characters 

 of the Terebratulidse united to the internal structure of the Spirifers, 

 to which latter family it truly belongs. Prof. Pliillips is the only 

 author who has recognized the group : he forms of it his last division 

 of the genus Spirifera, but gives no characters to distinguish it from 

 Terehratula ; the internal structure is, however, a sure guide." 



The above is all that he wrote about the genus at that time ; 

 and it will be perceived that he does not point out any parti- 

 cular species as the type, and, further, that there is nothing in 

 his remarks from which it can be inferred that he knew any- 

 thing about the genera into which the group was afterwards 

 subdivided. 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 clas- 

 sification advocated in 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 figure, 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. Speak- 

 ing of what he calls A. concentrica, he says : — " This species is 

 not uncommon ; it is figured in the ' Bull, de la Soc. Geol. de 

 Prance,^ with a perforated beak as in Terehratula. I have, how- 

 ever, seen numerous specimens with the beak entire and imper- 

 forate, as in the other palccozoic species." It is highly probable 

 from all this that he had in view such Silurian forms as A. tu- 

 mida. This latter species is so common that it is almost certain 



