808 DIVISIONS OF THE GASTROPODA. 



shell, the exterior of which is marked with chevron-shaped striae. 

 Possibly allied to the preceding types, but of wholly uncertain 

 affinities, are the genera Hemiceras and Salterella. In the first of 

 these are conical elongated shells, of circular section, in which the 

 walls are thickened by the deposition of concentric calcareous 

 lamellae, till only a small tubular space is left in the centre. The 

 genus is Ordovician. Salter ella, of the Upper Cambrian and Ordo- 

 vician rocks, comprises conical tubes, resembling the preceding in 

 shape, but consisting of several hollow cones placed one within the 

 other. 



Family 5. Conulariid^e. — This family includes only the re- 

 markable genus Conularia (fig. 723), the true relationships of 

 which have not been clearly established, though the genus is 

 usually placed among the Pteropoda. The shell in Conularia is 

 very thin, and is pyramidal in shape, the transverse section being 

 rhomboidal or four-sided. The apex is often deciduous, and the 

 internal cavity may be partitioned off near the apex by one or more 

 transverse plates. The aperture of the shell is contracted, and is 

 partially closed by tongue-like prolongations 

 from its corners. Each of the four faces of 

 the shell may be provided along its entire 

 length with an internal median longitudinal 

 fold, sometimes with two such, but these may 

 be wanting or rudimentary. Externally, each 

 face of the shell is divided into two equal 

 halves by a longitudinal groove, corresponding 

 with the internal fold above mentioned ; and 

 the surface is ornamented with transverse, 

 smooth, or tuberculated ridges, which are 

 angulated in the middle line of each face, so 

 as to form a series of obtuse angles, the apices 

 of which point towards the aperture. 



Fig. 723. — Conularia or- x 



nata. Devonian. I he genus Conularia attains its maximum 



in the Ordovician and Silurian rocks, some 

 of these early types attaining a length of nearly a foot, with a 

 breadth of more than an inch. The Devonian and Carboniferous 

 rocks have yielded a few species, and two forms occur in the Per- 

 mian, while a single species has been detected in the Trias, and 

 another has been recorded from the Lias. 



Family 6. TentaculitiDjE. — This family comprises only the 

 genus Te?it acuities, in which the shell (fig. 724) is comparatively 

 thick and solid, and has the form of a straight, elongated, conical 

 tube, from half a centimetre up to three centimetres in length, which 

 tapers to a closed apex at one end, and expands towards the other 

 to a circular aperture. The tube is always circular in outline, and 



