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Show 1878.] SHELLS OF CEPHALOPODS. 957 extravagant for adoption. And yet, if the Ammonite was tetra-branchiate like the Nautilus, no other conclusion could logically be drawn. Accordingly, such view is formally repudiated by the experienced zoologist Dr. J. E. G R A Y , in his paper " O n the Animal of the Spirula"1. "The examination," he writes, "of this animal confirms me in the opinion which I expressed in the * Synopsis of the British Museum' (1840, p. 149), that the Ammonites, from their texture and the small size of the last chamber, are internal shells and should be arranged with the decapodous Cephalopods, being chiefly distinguished from the Spirula by the siphon being always on the "dorsal margin of the whorls and the septa foliated on the edge. I am aware," he adds, "that this opinion is not in conformity with the ideas of many zoologists and comparative anatomists ; for Mr. Owen, in the last arrangement of these animals (Todd's Encyl. Comp. Anat.), though he places the Spirulcs with the Dibranchiate Cephalopods, places the Ammonites with the Tetr-abranchiate next to Nautilus" (p. 259). Before recapitulating the grounds on which it was inferred that the Ammonitidee were external shells, I will, finally, cite the valuable Memoirs of the Geological Survey of India, published under the direction of the accomplished and lamented Superintendent of the Survey, T H O M A S O L D H A M , LL.D., F.R.S., in which the sanction given to the views of Gray and before-cited authors as to the aspects of the shells of the Ammonites has mainly induced me to offer the present elucidation of the grounds on which I still hold the contrary opinion, viz., that the siphuncle in the Ammonitidee is "ventral " or " margino-ventral," as it is in Spirula, but thatit is " external" in the one and " internal " in the other, through the reversed direction of the spiral whorls. In the " Monograph of the Cretaceous Cephalopoda of Southern India," the preparation of which was confided to the accomplished naturalist F. STOLICZKA, too early lost to science, he premises the following characters of the Ammonitidee :- " Animal not known : shell spiral, more or less involute with numerous regularly (?) and gradually increasing whorls in the same plane, many-chambered, the last or body-chamber extending generally over about two thirds of the last whorl. Tbe margins of the septa are deeply divided into lobes and saddles, the first having their subdivisions always pointed, the latter more rounded. The dorsal lobe is divided by a small saddle into two parts, corresponding to the siphuncle: in the regular forms of Ammonites this is always placed in the middle of the back; the siphuncle is also similarly placed inside the shell." It is to be observed, however, that the author associates the Ammonitidee in the same order " T E T R A B R A N C H I A T A " with the Nautilidee ; and it would seem therefore that he entertained the doubts originally expressed by Gray2 and supported by Grant3 as 1 Annals and Magazine of Natural History, vol. xv. 1845. 2 Phil. Trans. 1833, p. 774. 3 M y description and figures were called in question, with more detail, by |
