FOURTH, OR CORONICERAN BRANCH. 167 



Arnioceras Hartmanni, Hyatt. 



Plate II. Fig. 17, 18. Plate III. Fig. 1, 1 a. Summ. PI. XII. Fig. 5. 



Aram. Hartmanni, Oppel, Der Jura, p. 79 ; Wiirt. Jahresh., XII. p. 199. 



Amm. geometricus, Oppel (pars), Ibid. 



Am. kridiforme, Hyatt, Bull. Mus. Comp. Zool., I., Jfo. 5, p. 74. 



Amm. kridion, D'Orb., Terr. Jurass. Ceph., p. 205, pi. li. 



Amm. falcaries (pars), Quenst., Amm. Schwab. Jura, pi. xiii. fig. 21. 



Amm. robustus (pars), Quenst., Ibid., fig. 22. 



Localities. — Whitby, Lyme Regis, Semur, Bonnert, Suabia, Gmiind, Adnet. 



This species has more compressed adult whorls than any variety of Am. semi- 

 costatum, and, although the pilae are similar, they begin to appear at an earlier 

 age and are developed more gradually. The abdomen is also in many speci- 

 mens, though not in all, distinctly channelled, and the keel prominent. 



In one specimen the superior lateral lobes are nearly as long as the abdominal 

 lobe on the sixth whorl, and on the same whorl in another they were two fifths 

 shorter. The inferior lateral saddles are deeper than the superior laterals, and 

 the inferior lateral lobes very short. 



A specimen in the Museum of Stuttgardt, collected by Prof. Fraas, is from 

 Arietenkalk, Hechingen (No. 5026 of that collection) ; another, from Gmiind, 

 was found in the Geometricus bed, and is labelled Amm. Nodolianas, D'Orb. Two 

 others, from Behla, are labelled Amm. falcaries, Quenst. ; these belong to his 

 sparsely ribbed variety, which is just intermediate between Am. Hartmanni, and 

 some varieties of Am. semicostatum. 



A specimen of falcaries, Quenstedt, figured by him, 1 is described as partly 

 unrolled. Examination of the broken end, however, shows that a calcareous 

 worm tube has occasioned the distortion by having been built upon the abdomen 

 of the growing shell. This is a common occurrence, and was observed in several 

 specimens at Semur. There are others, however, in which this distortion, as in 

 other species of Ammonitinas, takes place without the presence of any foreign 

 body between the whorls. Such examples have been named Crioceras Ert/on and 

 Mandubais by Reynes. These and other cases show the kind of error likely to 

 occur from the use of such names as Crioceras, Gyroceras, etc. 



The flattened sides and general aspect of Quenstedt's figure of falcaries ro- 

 bustus 2 agrees apparently more nearly with this species than any other known 

 to me. This species is the one commonly named Amm. geometricus, Oppel, in 

 the collections in Germany, and seems to have been in part confused with that 

 species by Oppel. 



1 Der Jura, pi. viii. fig. 6. 2 Amm. Schwab. Jura. 



