﻿BRITISH FOSSIL CEPHALOPODA. 55 



forms have usually shorter body-chambers and external siphuncles; and thus they 

 form so peculiar a group, that if the genus were to be split up, it would be advisable 

 to set these apart as distinct. The aperture is always simple ; though, as the shell 

 appears to have been thin, it not infrequently happens that the pressure of the rock 

 has forced the edges closer together than they would naturally be, and gives rise 

 to a false appearance of contraction. On well-preserved specimens there is often a 

 thickening of the shell below the aperture. 



The Cyrtocerata of the newer Palaeozoic rocks are often remarkable for their com- 

 plicated ornamentation, which runs in both longitudinal and transverse directions ; 

 but among the Silurian species ornament is rare, and is almost entirely confined 

 to transverse folds or riblets, and the shell is so thin that external and internal 

 surfaces are alike : some species show traces of colour. The distance of the septa is 

 usually slight, and the last two are often closer together than the rest ; they are of 

 slight convexity, and more often slant forward on the convex side, approximating to 

 the direction of the polar radii of their curvature, and have a slight natural con- 

 cavity forwards. The siphuncle is almost always small, so that, in spite of the close- 

 ness of the septa, the elements are usually longer than broad. They are, however, 

 almost invariably more or less inflated between the septa, and resemble beads or 

 invaginated cups. The siphuncle usually lies at the extremity of the diameter, either 

 on the convex or the concave side ; but by no means constantly so. These two 

 positions have given rise to the grouping of the genus, and they are interpreted to 

 mean, not that the siphuncle in one group arose from a different part of the body 

 to that of the other group ; but, taking the siphuncle as a fixed point, one group 

 had its curvature in the opposite direction to that of the other. 



Divisions. — The first division was proposed by De Koninck in 1844, who 

 groups them as: 1. Cyrtocerata phragmocerata ; 2. C. Icevia; 3. C. ornata. The 

 first of these groups, which for that author includes all species with an internal 

 siphuncle, must fall into some other, as it is impossible to include the Phragmo- 

 cerata in this genus. The other two groups are of importance, as the third may be 

 said to be nearly excluded from Silurian rocks. The Doctors Sandberger, in 1850, 

 also divided the Cyrtocerata, according to their ornaments, into four groups, — viz. 

 the smooth, the transversely ornamented, those with a network, and those with 

 longitudinal ornaments. G-iebel, in his ' Cephalopoda der Yorwelt,' made a primary 

 subdivision according to the shape of the section ; practically founding the subgenus 

 Trigonoceras, without giving it a name ; the remainder he subdivided into two 

 groups, according as their ornaments were transverse or longitudinal. Barrande 

 divides them into two series, according to the position of the siphuncle : I. Exo~ 

 gastric, in which that organ is external; II. Endogastric, in which it is internal. 

 In these two groups he includes those whose siphuncle, though near the centre, 

 inclines either one way or the other ; but it would be preferable to make a third 



