146 GENESIS OF THE AEIETID^E. 



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. 1 



The large form of Cal. 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 Cal. 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 raricostatum. The peculiar variety of tortile from Quedlin- 

 burg is one of these, but it has smooth and gibbous young whorls like the 

 young of Johnstoni. Cal. sulcatum, with coarse but sparse pilae, from Semur, 

 is another example. Some varieties of carusense afford other examples, but 

 raricostatus 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 pilae 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, Hyatt. 



Ariet. tardecrescens, Blake, Yorkshire Lias, p. 285, pi. 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 pilae, 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 pilae on the outer whorl have become depressed and curved. Specimens 

 in the British Museum from Robin Hood's Bay have been named Conybeari by 



1 Plate I. Fig. 25 a represents an abdominal view of the adult. 



