CEPHALOPODA. 11 



Order — Tetrabranchiata. 



Family — AMMONiTiDiE. 



Ammonites, Brugiere. 1789. 



Ophiopomorphites, Plott. 



Planorbites, Orbulites, Globites, Planulites, Lam. 



Amaltheus, Planulites, Be Montfort. 



Planites, Globites, Be Haan. 



Nautilus, Argonauta, ReinecJce. 



Ammonita, Orbulita (pars.), Fleming. 



A more or less discoidal, multilocular shell, with contiguous volutions; volutions 

 generally visible, septa transverse, with sinuated edges, perforated by a single tube, situated 

 close to the outer margin. 



Ammonites stjb-contractus. Plate II, figs. 1, \a, jun., figs. 2, 2a. 



A. Testa discoided, subglobosd, costatd, umbilicatd, anfractibus involutis, rotundatis 

 compressis, lateribus 16 — 18 costatis, costis obtusis bi-trifurcatis, in dorsum continuis ; 

 aperturd semiellipticd subcontractd ; umbilico magno, excavato, subconica. 



A sub-globose, deeply umbilicated, and costated shell, with sixteen to eighteen obtuse 

 ridges (tubercles ?) surrounding the margin of the umbilical cavity, from each of which 

 three or four smaller costse pass over the somewhat depressed and rounded back. 

 Aperture, semi-elliptical. 



Proportion of umbilicus to diameter, rather more than one-half. Diameter, 5 inches. 

 Thickness, 3 inches. Height of aperture, 1^ inches, twice as wide as it is high. 



The specimen from which our figure is taken has been much worn by clearing it 

 from the original matrix, but a careful examination discloses the prominent marginal costae, 

 as well as the smaller ones which arise from them and pass over the back. 



In the umbilicus, the marginal costse are well exhibited, which in the young state were 

 more compressed, and continued on the inner side of the cavity. 



This species is distinguished from the Amnion, coronatus, Brug., by its more globose 

 form, less conical umbilicus, and the more arched and less expanded aperture. It is closely 

 allied to Am. contractus, Sow., and in a young state might be mistaken for that species ; 

 but the ribs are larger and not so numerous or elevated ; the less embracing volutions, and 

 the more contracted form of the aperture in the adult shell, are also characters by which it 

 may be distinguished. 



Unfortunately the determination of the species, and their varieties of the Ammonites in 

 the Great Oolite of Minchinhampton, is rendered extremely difficult, in consequence of the 

 great rarity of specimens, and their state of preservation, rarely allowing the least trace of 

 the sinuated edges of the septa to be -observed. 



