Athyrisina squamosa var. rhomboidale no\. 

 PL XXIII., Figs. 7 and i8. 



In all essential characteristics this is coincident with the type species. For 

 example, the shell is extraordinarily thick, and is ornamented with very thick 

 radial striae on the surface. Median sinus and fold are eciually well developed. 

 The spirals are laterally directed. This variet}', however, is distinguished 

 from the type species by its outline which is somewhat more of the transverse 

 type. The type species is rather discoidal in form, the length and the width 

 not being very different. In this variety the hinge-line is no': appreciabh' 

 shorter than the maximum breadth of the shell which is just below it, and it 

 is also ciuite straight. The beak and the median sinus of the ventral valve, 

 however, being equall}- projecting as in the type species, the outline of the 

 shell is consetpiently obtusely rhomboidal ; hence the name rhomboidale, 

 This variety is also uniformK- thinner than the preceding species. Ventral 

 area is little better dex eloped in this form. The beak is somewhat obliquely 

 truncated by the pedicle opening. 



Dimensions : — 



Lengtli. Width. Thickness. Hinge-line. 



19 mm. 24 mm. 12 mm. 17 mm. 



19 » -^3 15 >. i'*^ 



20 ,, 26 „ 15 „ 20 „ 



25 „ 31 16 „ 24 „ 



Remarks : — The variet}- is no doubt different from the t}'pe species 

 in many points as mentioned above. In the material now at the writer's 

 disposal there is hardly an}' specimen that represents a type transitional be- 

 tween this and the preceding species. However, it may happen in future 

 that such forms will be found. In such case these two will of course be 

 united, and the varietal name will then be abandoned. To speak the truth, 

 there is only one specimen which appears to represent the t}'pe species 

 in all essential features, and yet it has a somewhat rhomboidal outline (PI. 

 XXIII., fig. 17). The writer, however, is not inclined to place it just between 

 these two forms because it is so very closel}- related to the type species. 



