156 GENESIS OF THE ARIETIML 



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 Psil. 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 Arietidae. 



Vermiceras spiratissinram, Htatt. 



Plate I. Fig. 17, 18. Summ. PI. XI. Fig. 23. 



Amm. spiratissimus, Qoenst., Handb. d. Petrefact., p. 355, pi. xxvii. fig. 9; Amm. Schwab. Jura, pi. xii. 



fig. 7-10; pi. xiii. fig. 1, 2, 6. 

 Discoceras spiratissimus, Hyatt, Bull. Mus. Comp. Zobl., I , No. 5, p. 77. 

 Amm. arietis, Ziet., Verst. Wiirt., p. 3, pi. ii. fig. 2, 3 (not fig. 4 1 ). 

 Amm. Conybeari, Ziet., Ibid., p. 35, pi. xxvi. fig. 2. 

 Amm. latisulcatus, Quenst., Amm. Schwab. Jura, pi. xii. fig. 1-4 (uot fig. 5, 6). 



Localities. — Whitby, Semur, Filder, Stuttgardt, Balingen, Vaihingen, Nellingen, Metzingen, Hohenheim. 



Var. A. The pilae begin early upon the third whorl, the shell during the first 

 two whorls being smooth. The pilae 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 Gal. 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 pilae gradually acquire their adult characteristics. 



The variety figured on Plate T. Fig. 18, might be called the dwarfed or rari- 

 costatas variety of this species. The pilse 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 

 spiratissimum. 



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 pilae are only ridges, destitute 

 of genicular and slightly bent forwards. The whorl is broader near the dorsum, 

 and the sides converge. The keel and channels remain apparently unchanged. 



1 This seems to be identical with carusense, judging from the type in Oppel's collection. 



