145 



valve. There is usually a faint indication of a mesial sinus in this valve, 

 which however only amounts to a flattening of the shell along the middle 

 of the lower two thirds. Dorsal valve moderately convex, most elevated 

 at one third the length from the beak ; umbo obtusely rounded, beak con- 

 cealed; hinge-line curved but distinct for two thirds the whole width. 

 Surface smooth or with many obscure accretion ridges ; in a few individ- 

 uals these latter are strongly developed, but in general the aspect is that 

 of a smooth shell. Length from 6 to 9 lines , width varying from two 

 thirds to eight ninths of the length. 



This species is more elongated than A. Prinstana, and has the umbo 

 of the ventral valve more strongly developed. The two species appear to 

 be connected by intermediate hnks ; but in a large number of specimens 

 two groups can be selected having upon the whole a distinct specific 

 aspect. 



Both are allied to the several species of this group found in the Clinton 

 and Niagara rocks of New York, and especially to A. naviformis (Hall), 

 which differs in being radiately striated and in having a deep mesial 

 sinus. 



Locality and Formation. — Junction Cliff, Anticosti. In Division 1, 

 Anticosti group ; Middle Silurian. 



Collector. — J. Richardson. 



Athyris Prinstana. (N. sp.) 



Fig. 122. a, b. 



Description. — Shell subcircular, moderately convex, smooth ; apical 

 angle about 105° ; cardinal slopes straight or nearly so for one fourth the 

 length of the shell, greatest width about the middle or a Httle in front 

 thereof ; sides gently convex ; front rather more narrowly rounded than 

 the sides. Ventral valve evenly convex, most elevated in the upper half; 

 the outline thence to the beak forming a smooth spiral curve ; a somewhat 

 straight or gently convex slope to the front margin ; umbo large rounded ; 

 beak small, closely incurved, very nearly if not absolutely in contact with 

 the umbo of the dorsal valve. Dorsal valve uniformly depressed convex, 

 most elevated at about one third the length from the beak ; umbo small, 

 rounded ; hinge-line nearly straight and equal to about two thirds the 

 whole width of the shell. Surface smooth. Length 7 lines ; width about 

 the same ; depth of both valves 5 lines. 



Locality and Formation. — Prinsta Bay, Anticosti. In Division 1, 

 Anticosti group ; Middle Silurian. 



Collector. — J. Richardson. 



