or 
LAMELLIBRANCHIATA. 53 
Oyrtodonta ] 
Upper Silurian forms now referred to it bring but impoverished remnants of the 
powerful stock that preceeded 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. breviuscula, canadensis, huronensis, rugosa, spinifera and subcarinata, all 
described by Billings, C obliqgua Meek and Worthen, and C. obtusa, saffordi, subangu- 
lata and subspatulata of Hall. The remainder belong to Whitella, Ortonella, Vanua- 
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 Conrad, 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 Palearca in which he proposed to include precisely the 
same group 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 
Palearca and “should an examination of the typical species prove the two identical 
the later name will give place to that of Cypricardites”. Winally in a supplemen- 
tory note to vol. iii (p. 524) he again uses this cut and now adopts Cypricardites in 
place of his Palwarca and Billings’ two genera Cyrtodonta and Vanuxemia. I 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 Palearca and the majority of the others 
Cyrtodonta. 
*The following belong to Wihitella: hindiand plebeia of Billings; megambonus and quadrangularis of Whitfield; sterlingensis 
~ Meek and Worthen; and ventricosa of Hall. The new geners Ortonella is founded upon C. hainesiS. A. Miller. OC, hayniana 
Safford, and niota, rectirostris and rotundata Hall, belong to Vanuxemia, while C. gantt and winchelli of Safford belong to the 
new genus Modiolodon. 4 
