166 GENESIS OF THE ARIETIDA. 
Var. C. 
The adults are remarkable for their prominent straight keels. The channels 
when present are very shallow. ‘'he young remain smooth for a longer time 
than in other varieties. The pile: appear between the first quarter of the fourth 
and first quarter of the fifth whorl. 
While smooth throughout the first three or four whorls, it is identical with 
the adults of miserabile, var. acutidorsale, but the sutures are more immature, The 
pilze develop quickly, and are similar to those of variety B. In fact, the young 
can be distinguished only by the larger size, quicker growth, and smaller num- 
ber of the pila. Associated with these are specimens with compressed whorls, 
perhaps males. 
Var. D. 
Plate Il. Fig. 15, 15 a 
This has a narrow abdomen, and the pilz are more prominent near the dor- 
sum. They appear on the fourth quarter of the third, or on the first half of the 
fourth whorl, earlier than in some specimens of variety B, and are more slowly 
developed. Some of the young are very like the adult of Arn. miserabile, var. 
cuneiforme. The pile bend forward when they first appear, or soon after, as in 
this variety, although subsequently becoming straight, like those of the adults of 
variety A. The abdomen is sharp, as in miserabile, var. acutidorsale ; the keel does 
not appear until the pile begin to acquire their prominent, straight adult char- 
acteristics. The sutures have serrated outlines. 
The abdominal lobe (Plate II. Fig. 15 a) may be equal to the superior lateral 
lobes, or one fourth shorter, and the siphonal saddle is remarkably large. The 
superior lateral saddles are divided symmetrically, and are equal in depth to the 
inferior laterals, The superior lateral lobes are very broad, and about one half 
longer than the inferior laterals. These characteristics were observed on the 
third quarter of the fifth volution of a specimen from Semur. In specimens from 
Whitby, on the second quarter of the same volution the sutures are similar in all 
respects, but the superior lateral lobes are at first equal to the abdominal lobe 
in successive sutures, and then slightly longer. At still earlier stages, when the 
marginal lobes first appear, the abdominal lobe is usually of the same depth as 
the superior laterals, or shallower (Plate II. Fig. 15 a). One specimen from Semur 
had sutures maintaining these immature proportions even on the latter part of 
the fifth whorl. 
There are several specimens from Spezia in Bronn’s collection, Museum of 
Comparative Zodlogy, labelled by Capelini Amm. subarietinus, Menegh. These seem 
to belong to this species, but are so compressed that it is difficult to make sure of 
the identification. The young are smooth until a very late stage, as is usual in 
semicostatum, and the pile and form are also similar. They are in the yellow 
clayey shale, and described as from Coregna near Spezia. 
