FOURTH, OR CORONICERAN BRANCH. 165 



Var. C. 



The whorls are stouter than in variety B, and acquire on the first quarter 

 of the fourth whorl, or later, a close resemblance to those of semicostatiim. The 

 inferior lateral saddles are considerably deeper than the superior laterals, the 

 inferior lateral lobes considerably shorter than the superior laterals, and pointed. 

 The sutures were observed upon the latter part of the fifth whorl. 



Second Subseries. 

 Arnioceras semicostatum, Hyatt. 



Plate II. Fig. 10-16. Suiiim. PI. XII. Fig. 4. 



Ainm. semicostatus, Simpson, Aram, of Yorkshire Lias, p. 51. 1 



Ariel, semicostatus, Wright, Lias Amra., pi. i. fig. 7 (not fig. 4, 5, 8). 



Am. semicostatum, Hyatt, Bull. Mus. Comp. Zobl., I., No. 5, p. 74. 



Localities. — Whitby, Semur, Basle, Spezia. 

 Var. A. 



Plate II. Fig. 10, 16. 



During the first four and three quarters or five volutions, the shell closely 

 resembles Am. miserabile, var. acutidorsale. After this the pilse appear. They are 

 at first broad and depressed, but possess the sharp definition of true pilse, and 

 terminate abruptly on the edge of the abdomen. The abdomen is rounded, and 

 the keel a distinct though depressed ridge. Its time of appearance could not be 

 determined, but it was plainly apparent on the second or third quarter of the 

 fifth whorl, and previous to this the aspect of the abdomen was precisely that of 

 miserabile, var. acutidorsale. 



Var. B. 



Plate II. Fig. 11-14. 



In this variety the young resemble very closely the adults of Am. miserabile, 

 var. acutidorsale. The pilse make their appearance earlier than in variety A, but, 

 while becoming more numerous, often retain their fold-like aspect, and terminate 

 abruptly near the abdomen. The keel appears about the same time as the pilag, 

 and may be either with or without slight channels. In some specimens the 

 whorls become stouter than usual in the adult, and the geniculse prominent. In 

 one specimen they are inclined posteriorly. 



In the Museum of Stuttgardt there are three specimens of this variety, one 

 from Behla and one from Muhlfingen (No. 4688), both in the Geometricus or 

 Upper Bucklandi bed. Another from Filder was found in the Angulatus bed. 



1 This name does not appear in the first edition of Morris's Catalogue, 1843, but is found in the second 

 edition, 1854, as Amm. semicostatum, Y. & B., Geol. Yorkshire, p. 257. This is an erroneous reference, since 

 no such species was described in that work. In the Museum of Yorkshire is a specimen with this name, and 

 it was described by Simpson in his Monograph of the Ammonites of the Yorkshire Lias, which was not cited 

 in Mr. Morris's first edition, though published in 1843. 



