LAMELLIBRANCHIATA. 535 



Oyrtodottta 1 



Upper Silurian forms now referred to it bring but impoverished remnants of the 

 powerful stock that proceeded them. 



Many species have been placed under Cyrtodonta or Cypricardites, which is 

 usually considered as identical, that have no right there. Thus of forty-nine species 

 classed as Cypricardites by S. A. Miller in the 1889 edition of his North Amer. Geol. 

 and Pal., only eleven can with reasonable certainty be said to belong to Cyrtodonta. 

 These are C. hreviuscula, canadensis, huronensis, rugosa, spinifera and suhcarinata, all 

 described by Billings, C ohliqua Meek and Worthen, and C. obtusa, saffordi, 'subangu- 

 lata and subspatulata of Hall. The remainder belong to Whitella, Ortonella, Vanux- 

 emia and Modiolodon, or-are too little known for positive generic placement.* 



To the eleven species mentioned we must add seven that have been described 

 since the publication of Mr. Miller's list; also fifteen new species, of which ten are 

 published in this work. This makes a total of twenty-six valid Lower Silurian 

 species positively known to have the characters of the genus as above defined. 

 Two Upper Silurian species, Modiolopsis dicteus Hall and M. primigenia Gonrad, sp., 

 also fall under Cyrtodonta. These have unusually thin shells but their hinges are 

 essentially as demanded for the genus. 



A few remarks are necessary to explain my adoption of Cyrtodonta instead of 

 Conrad's Cypricardites as the name for this genus. Conrad's name has seventeen 

 years priority over that proposed by Billings, but it was not until 1859 when Hall 

 reproduced a sketch of the hinge that had been overlooked among the manuscripts 

 left by Conrad that any adequate idea of his genus was possible. In the mean 

 time (1858) Billings proposed and fully illustrated his genus Cyrtodonta. In the 

 following year Hall published (in Pal. N. Y., vol. iii, p. 27, and 12th Rep. Reg. N. Y. 

 State. Mus., p. 10) his genus Palcearca in which he proposed to include precisely the 

 same grpup of shells. In the museum report mentioned (p. 13) Hall reproduces 

 Conrad's sketch of the hinge of Cypricardites with the remark that both the descrip- 

 tion and figure of that genus as given by Conrad correspond in many respects with 

 Palcearca and "should an examination of the typical species prove the two identical 

 the later name will give place to that of Cypricardites". Finally in a supplemen- 

 tory note to vol. iii (p. 524) he again uses this cut and now adopts Cypricardites in 

 place of his Palcearca and Billings' two genera Cyrtodonta and Vanuxemia. 1 have 

 not noticed that the Canadian geologists have given up the use of Cyrtodonta. In 

 the United States however, with a few exceptions all use Cypricardites instead, 

 while of European authors Bigsby adopted Palcearca and the majority of the others 

 Cyrtodonta. 



*The fallowing belong to Whitella: Mndl and plebeia of Billings; megambomis and quadrangulans of Whitfield; sterlingensU 

 Meek and Worthen; and ventricosa of Hall. The new geners OrfoneJta is founded upon C.haineslS. A. Miller. Chayniana 

 Safford, and niota, rectirostris and rotundala Hall, belong to Vanuxemia, while C. ganti and winohelli of Safford belong to the 

 new genus Modlolodtan. 



