146 GENESIS OF THE ARIETID. 
become more prominent as well as more widely separated and fewer in number 
than on the fifth whorl of this species or of Cal. Johnstoni.* 
The large form of Oal. carusense exhibits no sign of senility on the beginning 
of the ninth whorl, thus attaining a much larger size in its adult condition than 
the typical raricostatus, which often exhibits signs of senile decay upon the latter 
part of the seventh whorl. 
The abdominal lobe is somewhat longer than the superior laterals, and the 
inferior laterals shorter than, or about equal to, the superior laterals. The 
superior lateral saddles are very broad in proportion to their depth, as are also 
the inferior laterals, the latter being either equal to or rather deeper than the 
former. The first auxiliary saddles are, as usual, very variable in size and 
form, but when compared with the inferior laterals they are very much more 
prominent than in the adult of Cai. carusense. This seems to be the only marked 
difference between the sutures of these two species, and it is probably not very 
important. 
The true raricostatus from the Raricostatus bed, is rarely misnamed in collec- 
tions, but there are other forms of distinct species occurring earlier which are 
frequently misnamed raricostalum. The peculiar variety of ¢ortile from Quedlin- 
burg is one of these, but it has smooth and gibbous young whorls like the 
young of Johnstoni. Cal. suleatum, with coarse but sparse pile, from Semur, 
is another example. Some varieties of carusense afford other examples, but 
raricostalus is frequently almost inseparable from the young of carusense until 
the specimens are over 51 mm. in diameter. 
The adult of variety A of true raricostatum is almost precisely like carusense 
in those varieties which have very closely set pile in the young, as in Plate I. 
Fig. 16. Cal. raricostatum, var. A, therefore, seems to have arisen through an 
arrested development of carusense, and then subsequently to have given rise to 
the peculiar flattened typical whorls of variety B. 
Caloceras aplanatum, Hyarr. 
Ariet. tardecrescens, BLAKE, Yorkshire Lias, p. 285, pl. v. fig. 5 a, b. 
Locality. —- Whitby. 
This species is represented by a specimen which we were at a loss to 
dispose of until we read Blake’s description, and saw the figure. The latter is 
poor, but with the description it suffices, if one has a specimen in hand. The 
whorl in the young has a completely caloceran form and pile, which are similar 
to the young of Cal. raricostatum. It is in fact a much compressed, keeled, chan- 
nelled form of caloceras similar to Nodotianum.- It occurs in Blake’s Jamesoni bed 
of the Middle Lias, but is doubtless to be accounted for in the Raricostatus 
bed of Wright, which is included in Blake’s Jamesoni bed. It is discoidal, and 
the pile on the outer whorl have become depressed and curved. Specimens 
in the British Museum from Robin Hood’s Bay have been named Conybeart by 
1 Plate I. Fig. 25 a represents an abdominal view of the adult. 
te me — ilar 
5 mabe 
