ANTICOSTI ISLAND FAUNAS. 
33 
it into three small ridges of unequal size of which the outer 
overhangs the dental sockets and ends abruptly and free; the 
inner ridges small. The middle ridges are slightly the longest 
and bear the crura which converge toward the dorsal valve for 
about one-sixth the length of the shell, where two knob-like 
crural apophyses are developed and almost come in contact. 
At the origin of these apophyses the primary lamallae are 
abruptly recurved and develop the vertical spirals of which each 
has eight turns or less and has the apex directed toward the cen- 
tral area of the dorsal valve. 
The genotype is Lissatrypa atheroidea , the specific name 
being selected to call attention to the fact that the shell resembles 
an Athyris (. Athyris ; oidos, like). According to Professor 
Schuchert (personal communication) Atrypa pkoca (Salter) is 
also to be referred to this genus. 
Lissatrypa atheroidea new species. 
(Athyris, oidos, like). 
(Plate I, figures 11-15.) 
1882. Athyris lara Davidson (not Billings), Suppl. Sil. Brach., p. 121. 
1910, Nucleospira n. sp., Schuchert and Twenhofel, Bull. Geol. Soc. Am., 
vol. 21, p. 714. 
Shell with the characters of the genus; width 14 mm.; 
length 14 mm.; depth of both valves 7 mm. 
This shell is very apt to be mistaken for Whitfieldia ? lara 
(Billings), a mistake which has already been made. For final 
determination it is necessary to see the character of the spiral. 
W.? lara , however, has a somewhat more prominent ventral 
beak, is slightly larger, less often has the ventral sinus and dorsal 
fold and does not have a layered structure to the shell. These 
differences, however, can not be relied on, since there are many 
specimens which so far as external characters are concerned 
may be put in either species. 
Horizon and Locality. Silurian; Gun River (5), about a 
mile west of Jupiter river; Jupiter River (3-5), mouth of 
Jupiter river. 
The cotypes are in Peabody Museum. 
