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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 Cad. daqueum as quadragonal in section, with a 
keel, faint channels, and straight pile, tuberculated in one variety. This form 
was perpetuated in Cui. carusense of the Upper Bucklandi bed; the keel and ribs 
were, however, somewhat more highly developed in one variety of Cad. raricos- 
tatum. The deep narrow abdominal lobe, also a peculiar arietian characteristic, 
appeared in Cul. carusense, and was perpetuated in raricostatum; it was repro- 
duced at a very early age in the last species, and in Cal. Deffnert. The peculiar- 
ities of the straight or curved, fold-like, and crowded pile 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. Custagnolai 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 pile with arietian 
genicule. These characteristics were maintained throughout the series, be- 
coming more intense in Ver, Conybeari, 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. spiralissimum, 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 retain 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 Psi]. planorbe to the first form, Arn. miserabile, or the lower varieties 
of Arn. 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 Arn. semicostalum, also, the pile assumed in most 
varieties the peculiar straight, trenchant aspect, and the prominent and square 
geniculx, which are characteristic of this genus. In Arn. miserabile and semicos- 
datum the keelless, smooth form of Ps?l. planorbe, var. deve, 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 
ll 
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