MUSEUM OF COMPARATIVE ZOOLOGY. 87 



In some individuals the abdomen is crenulated, resembling in this respect 

 7t. Boblayei. 



Rhacoeeras Boblayei Hyatt. 



Amm. Boblayei D'Orb., Terr. Jurass., Cepli., p. 25, pi. 69. 



Loe. St. Thibault pres de Semur; Coll. Boucault. 



The character of the septa allies this species closely with Loscombi, and 

 the abdominal crenulations are of the same character as those of some in- 

 dividuals in that species. 



DACTYLOID^. 



This family includes the Planulati and part of the Macrocephali. 



Coeloceras* Hyatt. 



Pilas on the abdomen bifurcated ; lateral pila? single or bifurcated with 

 one external row of tubercles, occurring regularly on each pilae, or at 

 intervals on widely separated pihe. The young are very much flatter than 

 the adult, and the sides consequently very narrow. They are smooth for 

 the first one or two whorls, subsequently becoming tuberculated. 



The tubercles almost immediately spread, forming the pilas ; they may 

 enlarge and remain distinct, or become absorbed and disappear upon alter- 

 nate pilre. The abdomen remains perfectly smooth for some time after the 

 lateral pilas are developed, not acquiring the abdominal pilas until the third 

 whorl is reached. Septa close together and very intricate in the adult 

 Abdominal lobe broader and deeper than the superior lateral. The in- 

 ferior lateral is nearly the same in size, and both are unequally divided into 

 three shallow, minor lobes. Superior lateral cell lobiform and together 

 with the inferior lateral, unequally divided by two minor lobes. 



Coeloceras centaurus Hyatt. 



Amm. centaurus D'Orb., Terr. Jurass., Ceph., p. 266, pi. 76, fig. 3-6. 

 Loc. St. Amand, Semur, and Balingen ; Coll. L. de Koninck and M. 

 Boucault. 



Coeloceras pettos Hyatt. 



Amm. }>e(tos Quens'dt, Flotzge., p. 178. 

 Amm. pcltos Quens'dt, Der Jura, p. 135, pi. 16, fig. 14. 

 Amm. crcnatrtfi Ziet., Verst. Wiirt., pi. 1, fig. 4. 



Loc. Venarey, Milhaud, Balingen, Metzingen, Hinterweiler ; Coll. Bou- 

 cault, L. de Koninck, Prof. Bronn, and Dr. Krantz. 



* KoiXor, hollow. 



