AMICULA. 43 



without diagnosis ; no species mentioned). Type C. vestitiis Sowb. 

 — Symmetrogephyrus Midd., part, 1847. 



Valves almost covered by the extention of the girdle over them, 

 leaving only a small rounded or heart-shaped portion exposed at 

 the apex of each ; posterior borders of valves produced backward in 

 rounded lobes at each side, the lobes completely separated by a 

 posterior sinus having the tegmentum at its apex. Posterior valve 

 having a posterior sinus and one slit on each side. Girdle more 

 or less pilose, often having pore rows. 



The essential features of Amicula are its small exposed portion or 

 tegmentum, situated at the posterior edge, and not extending for- 

 ward to the sinus, its Mopaloid posterior valve, short contour and 

 short gills. 



Dall has divided the genus into two subgenera thus: 



Amicula Gray s. s. Gills median, type A. vestita. 



Chlamydotioncha Dall, Gills ambient, type A. amiculata. 



On account of the doubt attaching to the identity of Ch. amicula- 

 tus Pallas, we may well suspend judgment pending the receipt of 

 fuller data. 



The presence or absence of tuft-bearing pores is a very mutable 

 feature and of no specific or varietal valve in Amiula. As the same 

 has been shown to be true of 3Io2)alia (q. v.) Plaxiphora, etc., it need 

 occasion no surprise in this case. 



A. VESTITA Sowerby. PI. 8, figs. 23-26. 



Oval, rather elevated, the valves nearly covered by a brown (or 

 when young, yellow) skin continued upward from the girdle, but 

 their outlines are plainly visible through this integument. 



The small exposed portion of each median valve is broadly heart- 

 shaped, and situated at the posterior margin of the valve; it is 

 sculptured with strong concentric grooves and a more or less distinct 

 granulation. There is no diflTerentiation into areas. The exposed 

 portion of the posterior valve is heart-shaped, with the mucro incon- 

 spicuous, near but slightly behind the middle. 



Interior pure white. Anterior valve having 6-8 irregularly 

 spaced and unequal slits; posterior valve having a deep sinus 

 behind, and a single small mopaloid slit on each side. Jugal sinus 

 rather small; sutural laminoe rather less projecting forward than the 

 posterior rounded lobes on each side. 



