AMMONITES — ZOOLOGICAL, ETC. 



81 



way, differ so little from each other, that they would not be 

 sufficiently distinctive for Geologists, who were not able to 

 devote much time to the study of species, and further, this 

 mode of measurement does not shew the enormous diffe- 

 rence which exists externally between the species having the 

 whorls enveloped and those having them free. These last 

 considerations determined me to take as a base the external 

 and the internal size of the last whorl, relative to the entire 

 diameter, and which has afforded much wider limits of varia-- 

 tion, and shews in succession the differences which exist 

 amongst the species ; but to render it more complete, I have 

 always shewn the relation of this diameter to the envelop- 

 ment of the whorls, and scrupulously noticed it. 



If, in regard to the greater or less envelopment of the 

 whorls of the spire of Ammonites, I cast a rapid glance over 

 the species, I shall find great varieties. We see, in fact, in 

 certain species, the whorls simply in contact. {A. suhfim- 

 briatus, Honnoratianus, Lepidus, quadrisulcatus, striatisul- 

 catus, Strangularis, Juilleti) ; at other times the whorls gra- 

 dually become so far enveloped, as to conceal a quarter of 

 their bulk {A. Cryptoceras, fascicularis, Diivalianus, Lyell% 

 ^c). From this point they proceed, until only half their 

 bulk is uncovered {A. Leopoldinus, interruptus, tuberculatus^ 

 mammUlaris.). They are still more enveloped in some Am- 

 monites, where less than a quarter of the whorls is visible, 

 {A, Beaudanti, fortisulcatus, Belus, latidorsatus, ^c.) In 

 other species the umbilicus gradually narrows, the last whorl 

 envelops all the others, and the spire becomes entirely em~ 

 l)racing {A. pulchellus, semisulcatus, VeUed<2, Largilliertia- 

 nus, 3fc.). If we compare the two extremes, there is a com- 

 plete disparity of form, whilst there is an evident and un- 

 interrupted passage, from the whorls which are entirely free, 

 to those which are completely embracing, and this greater 

 or less envelopment of the whorls is not always connected 



VOL. I. NO. III. G 



