DIFFERENTIAL CHARACTERISTICS. 81 



took place independently of increase in the breadth of the whorl by growth, or 

 of increase in the involution of the successive species. 



The radical species of the laqueum subseries showed a completely arietian 

 form of whorl. This appears in Cal. laqueum as quadragonal in section, with a 

 keel, faint channels, and straight pilse, tuberculated in one variety. This form 

 was perpetuated in Cal. camsense of the Upper Bucklandi bed ; the keel and ribs 

 were, however, somewhat more highly developed in one variety of Cal. raricos- 

 tatum. The deep narrow abdominal lobe, also a peculiar arietian characteristic, 

 appeared in Cal. camsense, and was perpetuated in raricostatum ; it was repro- 

 duced at a very early age in the last species, and in Cal. Deffneri. The peculiar- 

 ities of the straight or curved, fold-like, and crowded pilge are differentials of 

 importance, which correlate with the other immature transitional characteristics 

 of this series. The series described in the chapter on Descriptions of Genera 

 and Species discovered in the Northeastern Alps shows that highly compressed 

 forms with acute abdomens occurred also in this genus. Cal. Castagnolai had 

 a tendency towards increase of involution, though this shell, and even the 

 extreme form abnormilobatum, must still be classed as discoidal. 



In the radical species of Vermiceras, Ver. spiratissimum, the whorl became 

 quadragonal with flattened sides and abdomen, channels, and pilte with arietian 

 genicuke. These characteristics were maintained throughout the series, be- 

 coming more intense in Ver. Comjbeari, and inherited at a very early nealogic 

 stage in Ver. ophioides. The shells remained discoidal, however, as in Caloceras, 

 even in the largest specimens. Looking back, we see that the radical species, 

 Cal. Johnstoni and laqueum, and Ver. spiratissimum, formed a series of proximate 

 radicals, in which there was a regular gradation in the intensity of expression of 

 the different characters after they were once introduced, culminating in the 

 quadragonal form and arietian sutures of spiratissimum. We could, therefore, 

 with perfect propriety associate these three forms in a distinct series, and they 

 would then be related by gradations parallel with those occurring in either Calo- 

 ceras or Vermiceras, though composed solely of radical species. This is possible 

 because of the discoidal forms of the species of the vermiceran branch of the 

 Plicatus Stock, all of which have numerous whorls, and re.tain the very long living 

 chambers, at least one volution in length, of the Psiloceran Stock. 



The differentials of the Levis Stock had a more abrupt beginning, the transi- 

 tions from Psil. planorbe to the first form, Am. miserabile, or the lower varieties 

 of Am. semicostatum, having been less complete, and the forms separated by a 

 certain interval of time. There was also a much quicker transition from the 

 helmet-shaped whorl to the quadragonal. This took place in the first species of 

 the first series, and this radical, whether the one or the other of the two men- 

 tioned, is keeled in adults. In Am. semicostatum, also, the pilae assumed in most 

 varieties the peculiar straight, trenchant aspect, and the prominent and square 

 geniculae, which are characteristic of this genus. In Am. miserabile and semicos- 

 tatum the keelless, smooth form of Psil. planorbe, var. leve, was retained so long in 

 the growth of some individuals that it became characteristic of some varieties, and 

 in other species of this series, though less important, it is always found as a 



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