115 



AMMONITES. 



AMMONITES. 



148 



The following are Von Buch's groups, with their prevalent geological 

 distribution : 



Arietea. The back is usually broad, and carinated (often a furrow 

 on each side of the keel) ; the ribs are simple and strong. The 

 sutural line formed upon the following general model : 



The group of Arietes, including A. Bucklandi, A. Conybcari, &c., 

 belongs almost wholly to the Lias formation. 



Falciferi. The back is narrow, acuminated to a sharp keel (no 

 furrow on its sides) ; the ribs are elegantly and siginoidally bent. 

 The sutures differ from those of Arietes, the dorsal sinus D being 

 much less deep, with diverging and not parallel sides ; the sinus L 

 is very much deeper, and there are three or four smaller ones, a a' 

 a", near the inner edge of the whorls. The latter whorls usually 

 embrace the preceding ones. 



These Ammonites are numerous in the Upper Lias and Lower Oolite 

 formations. A. Strartywaysii of Sowerby is.au example. (Amman lies 

 Walcotlii does not belong to this division.) 



Amaithei. The back is generally acute and keeled, the keel 

 generally crenated ; the ribs generally a little sigmoidal ; the latter 

 whorls embracing the preceding ones. The sutures are in general 

 form much like those of the last division, but more richly laciuiated 

 and foliaceoua 



This group belongs to the Upper Lias and Oolitic formations. 

 ^litea amaltheus of Schlottneim (A. Stokeiii, Sowerby) is an 

 example from the Lias. 



C'lju-icorni. The back broad, without a keel. The ribs simple, 

 straight, strong, and crossing the back. Inner whorls exposed. 

 The sutures often approach to those of the A rietes in respect of the 

 sinus D ; but the posterior edge of D, L, and 1 range on the same 

 line, and the undulations are all lower and less foliaceous than in the 

 Falciferi and Amalthei. Ammonites planicostatus (Sowerby) ia an 

 example. The species are common in the Lias. 



I'/'tiitilati. The back and sides rounded; no keel; the inner 

 volutions exposed. The ribs are often divided over the dorsal region. 

 The sutures are remarkably laciniated and complicated ; the sinus (L) 

 extremely deep, and generally trifurcate. 



The species occur commonly in the Lias and Oolitic formations. 

 iiita communis (Sowerby) is an example from the Lias; A. 

 'in (Sowerby) from the Coralline Oolite. 



lini-tati. The back is broad and not keeled ; the whorls often 

 quadrate : the ribs are simple on the sides, but divided over the 

 back, and generally bear a tubercle at the point of division. 

 Ammonites Darm (Sowerby) is an example from the Lias. 



Coronarii. The back without a keel, usually broader than the 

 sides ; the ribs are straight and simple on the sides, but divided into 

 two, three, or more, as they cross the back, and the point of division 

 is usually sharply tuberculate. The sutures resemble those of the 

 Planwlati. The species occur in the Oolites, as A. Humphrenawut 

 (Sow.) ; A. Goweriantu (Sow.) ; and in the Lias, as A. Bechei (Sow.). 



Macrocejihali. The back is without keel, and round and broad, and 

 the umbilicus deep. The ribs are straight on the umbilical face and 

 simple, but sometimes arched, and generally divided across the back. 

 The sutures resemble those of the J' iait are somewhat 



differently proportioned to the dorsal and umbilical surfaces. The 

 species occur in the Oolite and Chalk. The Ammonites sublcevis 

 (Sow.), "is a good example, from the Kelloways Rock. 



.niti. The back without a keel, often broader than the sides ; 

 ribs tuberculated on the sides. The inner whorls exposed. The 

 sutures have the dorsal sinus (D) large and deep, the lateral sinus (L) 

 widely removed from it and very deep, and somewhat trifurcate. 

 Occurs in the Lias and Oolite, and more plentifully in the Chalk. 

 Ammonites Bdkerue of Sowerby is an example. 



Ornati. The back flat or even hollow, narrow, and not keeled; 

 the broad sides joining it at a right angle, marked in general by a 

 row of small tubercles or the numerous fine ribs which cross the 

 back and toward the inner edge unite in parcels to form acute or 

 knotted ridges. (The old shells are often plain.) The sutures have 

 the dorsal sinus (D) shallow, the lateral (L) deep. The species are 

 almost confined to the Oxford Clay and Kelloways Rock : as A. 

 Calloritnti/i (Sow.) ; A. Duncani (Sow). ; A. yemmatus (Phil.), &c. 

 (Von Buch rightly separated from these the Dentati in his original 

 memoir, though they have been injudiciously reunited again.) 



Dentati. To this group we refer Ammonites splendens, A. lautus, 



it' . i if tlie Gault. The back is flat or concave, and 



margined by tubercles, or prominent ends of strong ribs, often united 



near the inner edge into tnberculated ridges, but not crossing the 



back. The sutures resemble in general form the preceding. 



Flexwisi. The back narrow, with borders tuberculated or serrated 

 by the terminations of the ribs, and in a young state with a tuber- 



HAl. HIST. DIV. VOL. I. 



culated keel. The ribs are ridged or tubercled near the inner edge. 

 This group is quoted from the upper Oolitic and Chalk formations, 

 and A. asper and A. flexuoms are examples.. \ 



The classification above sketched is very far from perfect. It is 

 diflicult to define the groups, when we pass from the typical to the 

 ordinary species, and there are many forms which refuse to be 

 included in the formulsc. Still it is an admirable sketch, and when 

 the Ammonitidie are fully developed, according to the principles thus 

 exemplified by Von Buch, we shall have them recognised, not as a 

 genus with subdivisions, but as a family including many genera. 

 (ITOrbigny's ' Palteontologie Franaise;' ' Annales des Sci. Nat.,' 1841, 

 N.S., xvi. p. 113, also (1829) xvii. 267 ; xviii. 417 ; xxix. 5.) 



Having given this sketch,- it will be necessary to meet the question 

 whether the Ammonites were external or internal shells. Cuvier and 

 Lamarck thought that they were internal. The former says (' Regne 

 Animal/ last edition), " The smalluess of the last chamber might 

 induce us to believe that, like the Spirula, they were internal shells." 

 Mr. Owen, in his arrangement above quoted, says, "Animal unknown, 

 presumed to resemble the Nautilus; shell external .... The last 

 chamber the largest and lodging the animal;" and probably this was 

 the actual state of things. Dr. Buckland, in his ' Bridgewater Treatise,' 

 says, " The srnallness of the outer chamber or place of lodgment for the 

 animal is advanced by Cuvier in favour of his opinion that Ammonites, 

 like the Spirula, were internal shells. This reason is probably founded 

 on observations made upon imperfect specimens. The outer chamber 

 of Ammonites is very seldom preserved hi a perfect state ; but when 

 this happens, it is found to bear at least as large a proportion to the 

 chambered part of the shell as the outer cell of the Nautilus Pompilius 

 bears to the chambered interior of that shell It often occupies more 



Ammonites obtttsus. a, J, c, d, outer chamber. 



than half, arid, in some cases, the whole circumference of the outer 

 whorl. This open chamber is not thin and feeble, like the long 

 anterior chamber of the Spirula, which is placed within the body of the 

 animal producing this shell, but is nearly of equal thickness with the 

 sides of the close chambers of the Ammonite." 



It should be remembered that the specimen is apparently imperfect 

 at the aperture. The siphon or tube of communication may be traced 

 from d, where it opens into the last or outer chamber, along the edge 



Ammonites rostrattw. 



