172 GENESIS OF THE ARIETID.E. 



smooth, as in Arnioceras, on the first three whorls, the pita being acquired only 

 on the last quarter of the third or first quarter of the fourth volution. They are 

 only about twenty-five in number on the fourth whorl, and gradually decrease in 

 number on subsequent whorls. 



The sutures on the second quarter of the sixth whorl show close affinity for 

 Am. semicostatiim. This resemblance ceases in a great measure after the beginning 

 of the fifth volution, when the piloe assume an aspect similar to those of Cod. rari- 

 costatum. The resemblance is due to the fact that the pita remain undeveloped ; 

 if they become more prominent, the shell would be more like semicostatiim. The 

 keel is also not so well developed as in semicostaium, and on this account resem- 

 bles that of a species of Caloceras. The abdominal lobe at this time is slightly 

 longer than the superior laterals, and these are about one half longer than the 

 inferior laterals. The superior lateral saddles are broad, shallow, and deeply 

 divided by only one marginal lobe. The inferior lateral saddles are tongue- 

 shaped, and slightly deeper than the superior laterals. The lobes and saddles 

 are serrated, and the first auxiliary saddle is very small. There are four speci- 

 mens in the Museum of Stuttgardt which entirely confirm these observations. 

 They are from Behla, and labelled Amm. kridion, Quenst, One is more com- 

 pressed than the other, and closely approximates to Am. semicostcdum ; in fact, 

 the young shell is quite as smooth as the gibbous variety of that species, and 

 with the exception of the abdomen, keel, and sutures it is identical with it. 



The original of Quenstedt's description from Bebenhausen, the type in Sua- 

 bia, bears out these remarks in every particular; but in his " Ammoniten des 

 Schwabischen Jura," Figure 7 has channels and a form not identical with others 

 of this species, all of which are similar to the young of Cor. kridion during the 

 adult stage. 



Arnioceras? Nevadanum, 1 Hyatt. 



Amm. Nevadamis, Gabb, Am. Journ. Conch. Philad , V., 1869, p. 6, pi. iii. fig. 1 ; IV- pi. xvi. 



This interesting form was found near Volcano, in Nevada, and probably 

 belongs to the Lower Lias. The young as figured by Gabb is smooth, and 

 the late stage at which the pita were introduced, their linear, straight aspect, 

 and crowded arrangement throughout the young whorls, are unquestionably 

 arnioceran. The sutures also, as figured in Volume IV. Plate XVI., have the 

 characteristic outlines of this genus. Nevertheless, the older whorls have the 

 form and proportions of Ver. Cow/bean', and are also tuberculated, the abdomen 

 being keeled and channelled. The abdomen has, however, much broader chan- 

 nels than any specimens of Vermiceras yet observed. The character of this part 

 is evidently not unlike that of the specimen in the Stuttgardt Museum described 

 in note to page 70 as occurring in the Angulatus bed and vet having channels 



1 The species associated with this by Gabb, under the name of Amm. Colfaxi (Am. Journ. Conch., 

 1869, V. p. 7, pi. iv. fig. 2, IV. pi. xvi.), and reported as found in the Lias on the western slope of the 

 Sierra Nevada near Colfax, was in poor condition, and was consequently so badly represented in the figure 

 as to be indeterminable. 



