OF CONCHOLOGY. 15l 



Subgenus Rictocyma, Dall.* 



Shell and hinge like Astarte. Sculpture consisting of broken, 

 nodulous waves, irregularly concentric, covered with a thick- 

 epidermis. Equivalve and nearly equilateral. 



The sculpture of this form is so very peculiar that I have 

 thought it as worthy of a section to itself, as Acila and similar 

 subgenera. It is externally like Ptyclwmya, Agassiz, from the 

 lower oolite, but the sculpture is essentially different. 



Type Rictocyma mirabilis, n. s. 



Shell small, subtrigonal, olivaceus, moderately thick. Urn- 

 bones nearly erect, prominent, pointed ; exterior covered with a 

 thick olive-green epidermis lightly marked with concentric 

 lines of growth. Interior livid translucent greenish, smooth. 

 Hinge as in Astarte but more equilateral than in most species of 

 that group. Anterior margin more produced than the posterior, 

 while the contrary is the case in the majority of species of 

 Astarte. Sculpture similar in both valves, consistsng of eight 

 or nine ridges, arranged in a generally concentric manner, but 

 having a squarish fluctuation in the middle of the valve toward 

 the umbo, one on each side of this toward the margin, and recurved 

 more or less toward the anterior and posterior margins. These 

 ridges are broken and non-continuous toward the sides and are 

 rudely nodulous toward their extremities, which do not reach 

 the anterior and posterior margins in most cases. 



Alt. -12, Ion. -16, Diam. -04 in. 



Habitat, North Harbor, Unga Island of the Shumagin group, 

 in 8 fathoms, muddv bottom, one living specimen, September, 

 1865, Dall. 



I have looked in vain for any species with which this might be 

 compared. It is, as far as I am able to discover, unique in 

 sculpture. A species of Ptychomya, referred by Agassiz to the 

 Myidce, described by Lycett (Ann. Mag. Nat. Hist, vi, p. 408, 

 pi. xi, f. 6, 1850) has no teeth on the hinge, and has the ridges 

 diverging from and forming a sharp angle, with a line passing 

 from the umbo to the medio-anterior ventral margin. The fact 

 of its having an edentulous hinge is sufficient to remove it from 

 this family, to which it was referred by D'Orbigny, and to re- 

 move any suspicion of its identity with the present subgenus. 



B From Pkxtcc, broken, and Kujuu., wave. 



