GENEALOGY. 59 



have any, though, as in the keelless tortilis form, the abdomen may become 

 elevated, or very slightly subangular. Wright's figure * admirably illustrates 

 such an individual. The Sironotus variety figured by D'Orbigny has a more 

 compressed form and an earlier development of the keel. 



Second Subseries. — Caloceras laqueum has many varieties. First, those which 

 develop the keel at a very late period of the growth, and grade into the third 

 variety of Cal. tortile. With these we find, as sub-varieties, some which, either 

 immediately before or at the time when the keel is developed, change by growth 

 the general form of the whorl. The abdomen may become elevated, as in the 

 first senile stage of some varieties of tortile, or depressed, assuming the aspect of 

 carnsense or spiratissimum. Secondly, those which develop a keel at a comparatively 

 early stage, and either retain the rounded sides, or become subquadragonal and 

 approximate in form and pilce to Ver. spiratissimum? 



The senile whorl had metamorphoses, which produced an elevated or narrow 

 abdomen, similar to that of Cal. tortile at the same stage, though in some varieties 

 the sides were flatter, and there is a nearer approximation to the true trigonal 

 outlines of the old of Vermiceras. 



The young of carusense 3 repeated the characteristics of the intermediate forms, 

 but generally produced the keel at earlier periods. These are also lowest in 

 geological position, and pass into other varieties, occurring later geologically, 

 which are of much larger size. In all these larger specimens 4 the form is notice- 

 ably subquadragonal in the adult, and also has a keel, and sometimes faint 

 channels. There is a tendency in old age to produce a rounded whorl, with an 

 elevated angular abdomen in the clinologic stage, resembling the same part in 

 the old age of the prominently keeled varieties of tortile and nodotianum. In 

 the large variety of carusense we also find some forms which in their clinologic 

 stage have flattened and convergent sides, with a keel and slight channels. In 

 other words, there are some specimens which show a tendency in old age to 

 change the subquadragonal form of the adult, very much as in the genus Ver- 

 miceras. In this species, also, the sutures were observed in one case of extreme 

 age to lose the differentiated proportions of the adult, and partially retrograde, 

 becoming similar to those of Psiloceras. 5 The young and the adult of many 

 specimens of the raricostatum variety of carusense 6 are inseparable from the same 

 stages in the extreme of variety a of raricostatum? with the exception perhaps of 

 slight differences in the marginal digitations. 



The typical raricostatum? however, is not similar to Cal. Liasicum, being ex- 

 tremely broad transversely, and having a very immature gibbous whorl, which can 

 be called subquadragonal only in variety b. In old age, 9 even the broad whorl of 

 the typical variety diminished in transverse diameter, the abdomen became more 

 elevated, and the keel and pilae obsolescent, until finally a fragment of the old 

 whorl cannot be distinguished from the same stage of Cal. tortile or nodotianum. 10 



1 Lias Amm., Pal. Soc, I. p. 316, pi. xvi. 2 Summ. PL xi. fig. 22. 



3 Summ. PL xi. fig. 15. 4 PI. ii. fig. 1-3. 



6 PL ii. fig. 3, 3 a. e PL i. fig. 10. 7 PL vi. fig. 15. 



8 PL i. fig. 25 a. » PL i. fig. 24, 25. 



10 Compare section of old nodotianum, pi. i. fig. 10, with section of old raricostatum, fig. 25. 



