Hyatt.] 34 [October 5, 



The Amm. brevispina figured by Sowerby, appears to be a different 

 species from this, one that shows more prominently the planicostan 

 pilae. In fact, Sowerby 's figure resembles closer what I have called 

 Microceras sinuosum (laticosta Sow.) than anything else. These 

 distinctions, and the geological gap which divides the two species, 

 induced Oppel to give it a new name. 



MICROCERAS. 



Microceras biferum. 



Turrillites Valdani D'Orb., Terr. Jurass. Ceph., pi. 42, figs. 1,3. 



Amm. bifer bispinosus Quenstedt, der Jura, p. 104, pi. 13, figs. 10, 

 11 and 13. 



Amm. polymorphic mixtus Quenstedt, der Jura, p. 128, pi. 15, 

 fig. 12. 



M. biferum Hyatt, Bull. Mus. Comp. Zool., no. 5, p. 80. 



This species has septa very distinct from those of D. confusum, 

 and they approximate more closely in their outlines perhaps to those 

 of Psiloceras psilonotum than to Caloceras torus or Ophioceras raricos- 

 tatum. This is due to the presence of a median cell in the superior 

 lateral lobes in both psilonotum and confusum, and the outlines of the 

 lobes and cells which are very similar also. The species is of small size 

 and may be readily distinguished from 0. raricostatum, with which 

 Quenstedt thinks it to be very closely allied. The young are not so 

 cylindrical as the young or adults of raricostatum, and above all they 

 are never flattened dorso-abdominally with bulging sides, as in the 

 typical 0. raricostatum. In fact, the abdomen in the young is con- 

 siderably more elevated, the whole shell being thicker and larger 

 in the young as well as in the adult, than Ophioc. raricostatum at the 

 same age or the representative species, D. confusum. Subsequently, 

 in many individuals, a much closer external similarity is brought 

 about, and this is especially remarkable when the planicostan variety 

 of the young raricostatum is compared with the adult of biferum. 



When fully developed, the species may or may not have two rows 

 of spines,as the pilae may extend into one large, undivided projection 

 which cannot be called a spine, but is only a prominent, truncated 

 pilum, with or without very slight points or spines at either extremity. 



Quenstedt remarks that these tuberculated varieties, when unsyrn- 

 metrical, correspond to D'Orbigny's Turrillites Valdani. Another 

 variety presents only smooth pilae, and these when unsymmetrical are, 

 according to Quenstedt, identical with Turr. Coynarti of D'Orbigny. 



