16 



ALLAN HANCOCK PACIFIC EXPEDITIONS 



VOL. 23 



Original diagnosis: Si distingue dei tipici Amussimn specialmente per 

 la mole minore e per le coste interne piu rare, non appiate, incrassate, 

 non raggiungenti il margine. Questo gruppo e largamente rappresentato 

 alio state vivente come mostra il "Report of the Lamellibranchiata 

 collect, by Challenger" dello Smith. E gruppo affinissimo ai Prope- 

 amussium coi quali anzi parmi collegarsi con forme ai passaggio a coi 

 quali a rigore si potrabbe anche reunire. 



Translation : It is distinguished from the typical Amussium especially 

 by the smaller umbo and by the internal ribs which are less numerous, 

 not flattened, thick, and not extending to the margin. This group is 

 largely represented in the living state as shown in the "Report . . . 

 Challenger" of Smith. It is a group very close to Propeamussiuin, with 

 which it seems to me to be allied because of the form of the hinge, and 

 with which, strictly speaking, it could even be united. 



Additional diagnosis: Shells thin, more convex than Propeamussium, 

 usually orbicular or nearly so, occasionally higher than long ; valves 

 seldom gaping laterally; right valve usually flexed ventrally; auricles 

 moderately to quite unequal ; byssal sinus from moderate size to near 

 obsolescence. Right valve usually with concentric ridges only, but in 

 several species radial only, or both ; left valve radially ridged in most 

 species, and ridges usually rather prominent ; fine concentric ridges 

 frequently present also; radial rows of vesicles only, in several species. 

 Internal lirae reaching almost to ventral margin in most species ; lirae 

 not present in davidsoni Dall, and frequently absent in hoskynsi Forbes. 



Remarks: Parvamussium differs from Propeamussium sd. chiefly 

 in the presence of a byssal sinus, inequality of the auricles, greater con- 

 vexity of the disk and, in all but one or two species, the stronger sculp- 

 ture of the left valve. The number and length of the internal lirae and 

 the lateral gape of the valves are features actually too variable to be of 

 value as criteria. 



Dall referred Variamussium to Propea?nussiu?n (1898, p. 698), 

 but nearly all subsequent authors have regarded the former unit as a 

 synonym of Parvamussium. Cossmann & Peyrot (1914, p. 105) referred 

 the type of Paramusiu?n, Amussimn dalli, to Variamussium', see Re- 

 marks under Propeamussiu?n. 



Ctenamusium is sj^nonymized here because Iredale's diagnostic cri- 

 teria actually serve to unite that unit with Parva?nussium. Also, its type, 

 Amusium thetidis, is clearly referable to the latter. 



From Iredale's diagnosis of Glyptamusium no apparent reason can be 

 seen for considering it distinct from either Propeamussium or Parv- 



