Hyatt.] 32 [May 20, 



Coeloceras pettos. 



There are two distinct varieties with numerous intermediate forms. 

 One has large pilse in the young, and the earlier stages are insepara- 

 ble from those of C. centauries; the other has smaller and finer pilse. 

 The development is often distinct when the adults exhibit no percep- 

 tible differences. On the other hand, a very broad abdomened 

 typical variety, or a very narrow adult t shell, may be produced from 

 young that are precisely similar. In Quenstedt's collection the broad 

 variety is found in the Davceibed, somewhat above the other. 



C. pettos and centaurus, according to Fraas' and Quenstedt's col- 

 lection, are found together in the same bed where alone centaurus 

 appears in Quenstedt's diagram. This, and numerous intermediate 

 forms, show that they are merely varieties of the same species 

 though it is convenient to retain their separate names. 



Coeloceras Desplacei. 



The Amm. acanthus D'Orb. is a form of this species which has no 

 tubercles on the cast, very slight ones on the shell, and pilse very reg- 

 ularly divided. A very fine specimen in D'Orbigny's collection 

 shows the old age. The spines become obsolete, and subsequently 

 they lose their divided pilse and stretch across the abdomen in par- 

 allel lines. At the same time, also, the size of the whorl decreases, 

 the abdomen losing in breadth until the sides are only gibbous near 

 the dorsal line, and then converge rapidly outwards. The breadth 

 of the dorsum also decreases so that a decided tendency to return to 

 a cylindrical smooth form of the whorl is manifested in old age. 



Coeloceras crassum. 



In D'Orbigny's collection a fine specimen of this form reaches a 

 larger size than any I have seen, nearly 9 mm. In this, also, the 

 tubercles are lost in old age, pilse become single on the abdomen, and 

 the involution becomes less, as described above for C. Desplacei. 

 The Amm. crassus of Quenstedt occurs in Lias e and t, and in Fraas' 

 collection there are representatives of this species which have no 

 tubercles; they are from Metzingen. 



Coeloceras mucronatum. 



This is merely a compressed form of C. crassum. 



Dactylioceras commune. 



This species occurs in the compressed specimens of Lias " s " of 

 Quenstedt, and in Lias " £," as shown by Quenstedt's and Fraas' col- 

 lections, in company with Holandrei. See also Quenstedt's remarks 

 on Amm. communis and crassus in " Der Jura," p. 251. 



