58 GENESIS OF THE AEIETID.E. 



catenata in varieties having the most discoidal forms and the pilae crossing the 

 abdomen without a channel would throw some light on this problem, we begged 

 Professor Emerson to give us some specimens of this species from the Markolden- 

 dorf basin for examination. After making several preparations of those kindly 

 sent in reply to this request, the inner whorls and all stages up to the adult were 

 studied. The young to the diameter of 6-8 mm. resemble closely the older 

 nealogic stages of Wcehn. curviornalum 1 and circacostatum, as figured by Wahner. 2 

 The pilae are slightly bent forward on the abdomen, are fold-like, as in Psiloceras, 

 and without the sharp bend and tongue-like forward projection which are the 

 primitive indications of a tendency to form a median ventral channel. This 

 tongue-like projection is formed in the next stage, and the pilae, which have in 

 the mean time become very sharply defined and prominent on .the sides and 

 abdomen, also exhibit a slight flattening or decided depression, in most forms, 

 along the median line of the abdomen. It is evident from these facts that 

 Wahner was right in considering the species of the weehneroceran series as tran- 

 sitions to Schlotheimia. 



Caloceran Series. 



This series is divisible into two subseries. 



First Subseries. — The direct connection of the plicated variety of Psil. 

 planorbe with Caloceras Johnstoni {torus, D'Orb.) has long been insisted upon by 

 Quenstedt, and in his collection the intermediate tjpes are found. There is 

 (1) aplicatus with whorls slightly narrower dorso-abdominally than is usual in this 

 variety, and somewhat more prominent folds; then (2) one with the same form, 

 but still narrower dorso-abdominally, and for this reason with a blunter and 

 rounder abdomen ; then (3) 3 a young one of this form precisely similar to the 

 Johnstoni. These show that Johnstoni is an offspring of the plicated planorbis, in 

 which the more gibbous sides, narrower whorls, and rounder, broader abdomens 

 of the young of that form are retained throughout life. 



From Johnstoni one can pass by gradations into Caloceras tortile* First, the 

 typical tortile, with young until a late stage, having rounded abdomens and the 

 aspect of the narrower and smoother forms of Johnstoni. These become angular 

 on the abdomen at various ages, without producing a true keel. Secondly, those 

 which introduce a slight keel upon the elevated abdomen, but which subse- 

 quently disappears in the increasing angularity of the abdomen in the senile 

 stages. Thirdly, those which introduce a keel, then a squared or quadragonal 

 form of whorl, like that of Caloceras laqueum. Fourthly, those which are of very 

 large size, similar in all their stages, except in the angular abdomen, to the 

 stouter forms of Johnstoni, and like these becoming rounded in extreme old age. 



The first variety grades into Col. Liasicuin, which has rounded, gibbous 

 whorls in the young. This has a keel only at a very late stage, or may not 



1 Summ. PI. xi. fig. 7. 



2 Unt. Lias, Mojsis. et Neura., Beitr., XII., PI. xv., xvi. 



3 The larger one (2) was fortunately a broken specimen, and showed precisely the same form as (3) in 

 the young. 



4 Summ. PI. xi. fig. 14; PI. i. fig. 12-14. 



