1870.] 39 [Hyatt. 



tions by a pointed minor lobe, the depth of the superior lateral lobes 

 about equal to the abdominal lobe, and the shallowness of the inferior 

 lateral lobes, together with the great breadth of the cells and sim- 

 plicity of outline of the cells, and absence of numerous minor lobes 

 and cells, are all characteristics of the Arietida?. 



They show that planicosta, and the series to which it belongs, come 

 nearest to this family ; in fact, are precisely intermediate between the 

 Microceran series and the Arietidae. If, indeed, specimens of D. con- 

 fusum sometimes have a keel as stated by Quenstedt, the evidence 

 is still stronger. In the adult the triplicate division of the base of 

 the superior lateral cells, and the outlines of the septa, remind us 

 forcibly of Caloceras torus, the lowest of the Arietidae, though the 

 shallowness of the inferior lateral lobes still remains. This, however, 

 is probably sometimes found in C. torus and in those specimens in 

 which the development of the pilse is retarded, an external similarity 

 to the smooth abdomen and fold-like lateral pilae of C. torus is also 

 produced. 



Deroceras confusum. 



Amm. confusus Quenstedt, der Jura, p. 127, pi. 75, fig. 89. 



" planicosta Sow., (pars) Min. Conch., vol. iv, p. 149, pi. 

 406, not 73. 



Microceras confusum Hyatt, Bull. Mus. Comp. Zoology, no. 5, 

 p. 80. 



Amm. Lolibergensis Emerson, Die Liasmulde von Markoldendorf, 

 p. 61, pi. 3, fig. 3. 



In this species the first three and a half whorls are smooth and 

 flattened ventrally, the sides bulging as in 0. raricostatum. This 

 resemblance is still further increased by the development of the 

 pilae. On the latter part of the fourth, fifth and sixth whorls the 

 resemblance to raricostatum is very close, or rather to the earlier 

 stages of that species before the keel appears. On the sixth whorl the 

 tubercles begin to appear and the form changes to a more laterally 

 compressed and thinner whorl, and the tuberculated pilae cross the 

 abdomen as in the typical planicosta. 



The septa on the fifth whorl are quite like those of raricostatum 

 in their outlines, though the inferior auxiliary lobes and cells slope 

 inwardly and posteriorly. All the shells examined were small, 

 hardly more than an inch in diameter. The developmental resem- 

 blance to Q. raricostatum does not extend to the septa- These have 

 a close similarity to those of Caloceras torus, differing however in one 



