﻿BRITISH FOSSIL CEPHALOPODA. 225 



Trochoceras rapax, Barrande, PI. XXX. figs. 2, 2a. 



1865. Trochoceras rapax, Barrande, 'Syst. Silur. de Boheme,' vol. ii. pi. 21, 22, p. 124. 

 1877. „ „ Barrande, loc. cit., Supplement, pi. 493, p. 89. 



Syn. 1865. Trochoceras pmgue, Barrande, loc. cit., pi. 17, fig. 5, p. 112. 



Type. — Fragments only of this species are described by its author. They are 

 of large size, and consist of the later septal- and body-chambers ; the rate of increase 

 cannot therefore be defined, and the transverse growth as seen in the sections 

 is not uniform, being more rapid in the earlier portion. In the body-chamber 

 it is 1 in 14, measured along the outer curve. There is very little sign of 

 asymmetry. From the manner in which the ornaments pass over the inside, it 

 must have been evolute throughout the portions preserved. The section is very 

 roundedly quadrate, the chief flattening being on the front. In the earlier part 

 the diameters are very nearly equal, but it becomes more transverse with age. The 

 ornaments are strong ribs, which curve backwards and make a deep sinus on the 

 front, where they break up into finer ones ; they pass horizontally across the 

 concave side without becoming more feeble. They are about \ the mean diameter 

 apart in the middle of the side. There are minor ornaments formed by fine 

 longitudinal and transverse raised lines and epidermids on the cast. The ribs 

 become feebler towards the aperture, which is oblique in the same direction, but at 

 a greater angle than the ribs, and thus there is a deep sinus on the front, but 

 no contraction. The septa are moderately convex, but the sutures are not very 

 concave, but have so much of a forward curvature towards the outside as to cut 

 across four of the ribs ; they are four in number to every three ribs ; the siphuncle 

 is a little beyond the centre towards the outside. The largest is about 10 inches 

 long. From the band E 2, or lower part of the 3rd Fauna, or Upper Silurian. 



General Description. — Two very admirably preserved examples, one in the 

 British Museum and the other in the collection of Dr. G-rindrod, give certain proof of 

 the presence of a Trochoceras, in which the earlier whorl leaves a very wide interval 

 between itself and the body-whorl, and which therefore is evolute for some distance 

 at least previous to the last chamber. Measurements of the rates of increase show 

 that it becomes much greater with the growth, varying indeed from 1.22 to 1.9, and 

 the last whorl varies from .23 to .31 of the whole. From this we may expect that 

 the earlier whorls are in contact. The section as seen in Dr. Grindrod's specimen is 

 roundedly quadrate, rather flattened on the outside (fig. 2), and the two dimensions 

 are equal. In a younger example it is a little thicker than broad, with the greatest 

 thickness near the inside. The ornaments are well-marked, separate ribs, slightly 

 convex forwards, but curving rapidly back on the whole so as to make a sinus on the 

 front ; they are equally conspicuous all round, but feeble on the cast. There are 36 of 



2 G 



