156 GENESIS OF THE ARIETIDA: 
in which the channels were entirely obsolete, the keel being almost completely 
merged in the surface, or represented only by a raised sub-angular line on the 
sub-acute abdomen. 
These specimens are among the few rare examples of species in this family 
which have entered upon what we have called the nostologic stage on account of 
the resemblances to ancestral forms which make their appearance in consequence 
of senile degeneration of the differential characters of the preceding adult stages. 
It is to be anticipated that in very exceptional cases of extreme senility, the 
reversion to the form and characteristics of Psi. planorbe may have been com- 
pleted by the entire loss of the keel at the termination of the nostologic stage, 
but we have not yet seen such a case in Vermiceras, or any other keeled and 
channelled genus of the normal Arietide. 
Vermiceras spiratissimum, Hyarr. 
Plate I. Fig. 17,18. Summ. Pl. XI. Fig. 23. 
Amm. spiratissimus, QuENsT., Handb. d. Petrefact., p. 355, pl. xxvii. fig. 9; Amm. Schwab. Jura, pl. xii. 
fig. 7-10; pl. xiii. fig. 1, 2, 6. 
Discoceras spiratissimus, Hyarr, Bull. Mus. Comp. Zool., I., No. 5, p. 77. 
Amm. arietis, Zimr., Verst. Wiirt., p. 3, pl. ii. fig. 2, 3 (not fig. 41). 
Amm. Conybeari, Ziev., Ibid., p. 35, pl. xxvi. fig. 2. 
Amma. latisulcatus, Quenst., Amm. Schwab. Jura, pl. xii. fig. 1-4 (not fig. 5, 6). 
Localities. —Whitby, Semur, Filder, Stuttgardt, Balingen, Vaihingen, Nellingen, Metzingen, Hohenheim. 
var. A. The pile begin early upon the third whorl, the shell during the first 
two whorls being smooth. The pil are about twenty in number on the third or 
fourth whorl, and increase to from thirty-two to thirty-four on the sixth whorl, 
and forty-five to fifty on the eighth. The aspect of the shell, until the keel be- 
comes well defined, is precisely like that of the adult of Cal. laqueum. At earlier 
periods the development is the same as in that species; the keel, however, is always 
developed earlier. The whorls assume the flattened sides and abdomen on the 
fourth whorl, and this continues in some specimens throughout the fifth. The 
channels become deeper upon the last quarter of the fifth whorl or the first 
quarter of the sixth; but while these remain without well defined lateral ridges, 
the shell continues immature. When, however, the ridges appear, the abdomen 
and the pile gradually acquire their adult characteristics. 
The variety figured on Plate I. Fig. 18, might be called the dwarfed or rari- 
costatus variety of this species. The pile are between fifteen and nineteen in 
number on the third whorl, and only about twenty-two on the fourth, increasing 
again to twenty-six on the fifth. The young, however, are like those of typical 
spratissimum. 
Senile characters made their appearance in one specimen upon the ninth 
whorl. A fragment of the fourth quarter of this whorl, in one specimen, exhib- 
ited unmistakable signs of advanced senility. The pile are only ridges, destitute 
of genicule and slightly bent forwards. The whorl is broader near the dorsum, 
and the sides converge. The keel and channels remain apparently unchanged. 
i This seems to be identical with carusense, judging from the type in Oppel’s collection. 
