LECTURES ON MOLLUSCA. 21 



In the Lituites of the ancient seas, we have a Nautilus, which, on 

 coming to maturity, produced its tube in a straight line. The Hortolus 

 resembles it, but with the whirls separate as in Spirula. In TrocJioceraSj 

 we find the spire more or less elevated, as in snails. 



The sub-family CLYMENIDJE consists of forms in which the chambers 

 are more or less waved or indented, forming a slight approach to the 

 Ammonites. They are all palaeozoic forms, except Aturia, which makes 

 its appearance unexpectedly in the London Clay. This has a very large 

 internal siphon, like a number of funnels interwrapping each other, 

 and reminding us somewhat of Endoceras among the Orthoceratites. 



Family AMMONITID^E. (Bam's-Horn Shells.) 



This group, so abundant in the middle ages both in species and in 

 individuals, suddenly passed out of existence at the close of the creta- 

 ceous age. The body of the Ammonites was long in proportion : the 

 opening of the shell was guarded by curiously-shaped processes, and 

 closed by a double operculum. In the beautiful flat Ammonites of the 

 Oxford Clay, the shell makes two long forceps-shaped beaks, one on 

 each side of the mouth. In another species, these beaks arch over the 

 mouth and meet in the middle, leaving one hole for the head to crawl 

 out at, and the other for the opercle-bearing arms. In other species, 

 the aperture is almost closed up, as in many snails. 



In the keeled species, the operculum was of one horny piece, as in 

 Gasteropods : but in the round-backed groups, it was shelly, and divided 

 into two plates. Forty-five kinds have been described, one being from 

 the paleozoic rocks. They were called Trigonellites by the old writers, 

 and doctors still disagree as to their nature. D'Orbigny thought them 

 cirripedes : Meyer, bivalve shells : Sowerby, fish palates : Deshayes, 

 gizzards of Ammonites : Coquand (followed by Chenu) cuttle bones. 

 They have however sometimes been found in situ, exactly answering 

 to the hood of the Nautilus. 



But the most remarkable character of the Ammonites is the sutures, 

 or edges of the chambers. When an Ammonite is sliced down the 

 middle, the septa simply appear waved as in Clymene. But when the 

 outer shell is removed, and the cast of the edges is displayed, we find 

 a beautiful leafy structure, often of very intricate pattern, but constant 

 in each species. The siphuncle is always external. The outside is 

 almost always very beautifully ornamented, with ribs, knobs, spines, or 

 delicate stria?. The under layer is always pearly, as in Nautilus ; and 

 beautiful objects they must indeed have been, when painted with vari- 

 ous colors and patterns, to those who could have seen them with 

 oolitic or cretaceous eyes. Some of them are of enormous size, meas- 

 uring occasionally two feet in diameter. These are found in the Lias, 

 and in the neighborhood of Bristol (England) may often be seen built 

 into the walls by the road side. More than five hundred and thirty 

 species are already known. They are rare in America, but very com- 

 mon in Europe. Species, similar to those of the English oolite, have 

 been found in the high passes of the Himalaya, more than 16,000 feet 

 above the level of the sea. 



The most ancient of the tribe are the Gomatites, of the Upper Silu- 



