ARIETITES COLLENOTII. 305 



bounded by a sharp carinated ridge, produced by the union of the costal termination, and 

 forming a well-defined limit to the sulci. These marginal carina? are well seen on the 

 cast, but much better when the shell is preserved. 



In the fine large specimen, PI. XXII b, which measures 140 millimetres in diameter, 

 the ribs begin to disappear from the last half of the whorl, whilst they are well 

 developed upon all the inner turns of the spire. The keel is very sharp, the sulci broad, 

 and the siphonal lobe-line is seen in situ in fig. 3 ; the sagittal shape of the aperture is 

 well seen in fig. 2, and the structure of the lobe-line in fig. 1. The septa (PI. XXII A, 

 fig. 9) consist of three lobes and three saddles. The siphonal lobe is much longer and 

 wider than the principal lateral ; it has two branches, the sides of which are orna- 

 mented with numerous bilateral digitations. The siphonal saddle (PI. XXII b, figs. 1, 3) 

 is as large as the principal lateral lobe, and provided with numerous rounded festoons. 

 The principal lateral lobe curves outwards, has three digitations on its border, and termi- 

 nates in several points ; the lateral saddle is larger than the principal lateral lobe, and 

 terminates in numerous festoons ; the lateral lobe is divided into two parts, and each 

 has several digitations ; the lateral saddle is wide and shallow, and ends in six festoons ; 

 the auxiliary lobe is small, and terminates in two short branches. 



In PI. XXII a, fig. 9, I have given an enlarged drawing of the lobes and saddles of 

 the specimen figured in No. G, which agrees very well with the lobe-lines on the specimen 

 drawn in PI. XXII b, fig. 1, in which a line passing from the point of the siphonal lobe 

 to the centre passes beneath all the other lobes. 



Affinities and Differences. — This Ammonite very much resembles A. impendens ; its 

 keel, however, is larger, the last whorl is deeper, and it wants the overhanging projection 

 over the umbilicus so characteristic of impendens. Notwithstanding these differences, 

 A. impendens may be only a variety of A. Collenotii. It has some resemblance to 

 Arietites Brooki, still the depth of the outer whorl and the absence of ribs in old 

 specimens prove the distinctness of the species. 



A. Collenotii is a very remarkable species, as was long ago remarked by Professor 

 Alcide d'Orbigny, who observed that it united in one form two characters, in general 

 widely separated from each other. In its general form it resembles Harpoceras, and in 

 the structure of its lobes and saddles its affinities lie with Arietites. It is certainly very 

 distinct from all the species of both groups, with which it has affinities and differences 

 both interesting and remarkable. In its general form and structure, Arietites Collenotii 

 resembles A. Aballoensis, d'Orb., but differs from the latter in having its prominent keel 

 persistent on the cast, whilst in A. Aballoensis the keel exists only in the shell, and is 

 entirely absent in specimens denuded of the carina ; it likewise wants the lateral furrows 

 so characteristic of A. Collenotii. 



Dr. Oppel erroneously considered A. Collenotii to be a young individual of A. 

 Guibalianus, and that both species should be united. It will be shown in the sequel, 



40 



