MOLLUSCA (except CEPHALOPODA). 141 



few from Tyrone, and a fine series of the loosely-coiled Gallery 

 Maclurea from Scotland (Fig. 75 e). VIII. 



Cambrian Mollusca are few, and in Wales, where the Table-ease 

 rocks are exposed, the shells are poorly preserved. Glyptarca 

 primaeva and Bellerophon camhriensis (Fig. 75 c) may be noted. 



In a special Table-case at this end of the Gallery are placed Central 

 together the British and foreign specimens of CONULARIDA. 

 These, which are almost exclusively Palaeozoic, are some- 

 times regarded as forming a Sub- Order of Pteropoda ; but 

 now that the origin of the Pteropoda from Opisthobranch 

 Gastropoda appears certain, this view is scarcely tenable. 

 Some of the simplest forms are found in the Lower Cambrian 

 of North America, and of these Saltcrclla, Heleiiia, and 

 Hyolitlielhis are exhibited. They are thick-walled tubes, 

 straight or bent, smooth or striated, ending in a point, and 

 are generally placed in a family with Torclklla from Sweden, 

 which has a shell of phosphate of lime. Salterella has also 

 been found in West Australia, and HyolitlielliLs occurs in 

 Ordovician rocks. The Devonian Coleoprion is probably an 

 allied form. The shell of Hyolithes [Theca ] and a few similar 

 forms is composed of carbonate of lime, is conical, straight, 

 or curved, triangular, elliptical, or flattened in cross section. 

 Its surface is smooth or striated, and it is closed by an 

 operculum. The narrow end of the shell is often divided by 

 cross partitions. Species of this genus lived throughout the 

 Palaeozoic Era, but were most abundant in its earlier half. 

 The shell of Tentcmdites is an elongate cone, ornamented 

 with rings. It often begins in a small bulb, and the earlier 

 portion may be partitioned as in Hyolithes. Its thick wall is 

 composed of two layers. The genus abounds in Silurian and 

 Devonian rocks, often 

 being the chief consti- 

 tuent of certain beds. 

 Comdaria is the most 

 widely distributed in 

 space and time, and con- 

 tains the greatest number 

 of species and the largest 

 individuals of all Conu- 

 larida. The shell is elon- 

 gate and four-sided. Each 

 face is divided lengthwise 

 by a groove (Fig. 76 c) and 



ornamented with parallel ridges, which slope upwards toward 



a h c 



Fig. 76. — Conularia qitadrisidcata, Coal 

 Measures of Coalbrookdale. The lower 

 end is broken off. a, side view; b, 

 aperture seen from above ; c, back view. 



