120 GENESIS OF THE ARLETID.E. 



DESCRIPTIONS OF GENERA AND SPECIES OF ARIETID^E. 1 

 RADICAL STOCK. 



FIRST, OR PSILOCERAN BRANCH. 

 PSILOCERAS. 



SHELL smooth, plicated or with fold-like pilte in some subseries. The abdo- 

 men is rounded, or with smooth median zone, never channelled or keeled. 

 Whorl in section is compressed, helmet-shaped. The sutures are similar in pro- 

 portions and outlines to those of Caloceras. This is shown in the broad abdomi- 

 nal lobe and large siphonal saddle, the equality in length and size of the abdomi- 

 nal and lateral lobes and saddles, their leaf-shaped marginal digitations, and the 

 number and inclination posteriorly of the auxiliary lobes and saddles. 



The living chamber is one, or more than one, volution in length, and is shorter 

 in the young than in the adult stages. 2 Senility is indicated by increasing con- 

 vergence of the sides, and the loss of plications, 3 but a subacute abdomen, such as 

 appears in the old whorl of Wsehneroeeras, is never present. The completeness 

 and accuracy of Warmer's illustrations and descriptions, which enable one to study 

 all the stages of growth in some species, has tempted us to suggest the existence 

 of three subseries in this genus. (1.) The first contains smooth shells, typical 

 helmet-shaped whorl, and an old age in which a subacute whorl is not }'et re- 

 corded in any species. (2.) The second contains plicated shells exactly similar 

 in form, but the folds numerous and regular, and in some species figured b^y 

 Wanner these cross the abdomen with a forward bend. They are. however, not 

 true pilse, and, so far as we know, they do not become depressed along the median 

 zone as in Wadmeroceras. (3.) The third contains shells having psiloceran forms 

 but flattened sides, and often plicated as in the second subseries, though the 

 Psil. Hagenowii is smooth. We regard this subseries as of doubtful utility, but do 

 not know how to dispose at present of the forms it contains. 



1 Throughout this chapter there is no attempt to give a complete synonymy of any one species. 

 The references given under each name are only those which were considered essential to settle the applica- 

 tion of the specific name and the range of the forms to which it was applied in tins memoir. The localities 



;ie those of specimens in the collections of the Museum of Comparative Zoology. 



2 Quenstedt, Amm. Schwab. Jura, pi. i. fig. 6, shows a nealogic stage in which this chamber is not quite 

 half a volution in length. Wahner takes note of this. (Unter. Lias d. nordbst. Alpen, Mojsis. et Xeum., 

 Beitr., IV.. lSSil, p. 135,) and states that in one example of Psil. planorbe from Wurtemburg observed by him 

 the living chamber was only two thirds of a volution in length, and suggests the same opinion with regard 

 to the shorter living chambers of the yonng. 



:l Quenstedt figures what may be a fragment of an old specimen of Psil. planorbe, Amm. Schwab. Jura, 

 pi. iii. fig. 1, and Wahner has figured several old specimens in Unter. Lias, Mojsis. et Xeum., Beitr. 



