"754 ON COiNULARIA AND HYOLITHES, 



position, a shelly plate which would just fib the aperture." In 

 thus possessing an operculum Hyolithes departs from the structure 

 of the straight or slightly curved Pteropods, in which opercula 

 are rare, and approaches the spirally-rolled forms, many of which 

 are furnished with one, and it therefore occupies a very marked 

 position in the Thecosoinate division of the Pteropoda. For this 

 Theca-li^e, shell with an operculum Salter adopted the sub-generic 

 name of Cleidotheca. 



The description of Hyolithes lanceolatus by Morris is as 

 follows : — " Shell elongate, gradually tapering ; section obtusely 

 trigonal ; surface marked with numerous transverse striae, which 

 become arched as they pass over the posterior (^) portion of the 

 shell." 



The general form of the shell in Hyolithes is elongately pyram- 

 idal, usually curved, but occasionally straight. The curvature 

 occurs in one of two directions — either in the plane of the broader 

 faces or laterally. The transverse section is triangular — either 

 rectilinear or curvilinear. The faces of the pyramid are usually 

 plain, but at times marked by longitudinal ridges. The aperture 

 is generally oblique to the longer axis, and in some a segment of a 

 circle, often semicircular. The sides, or lesser faces, are acute or 

 rounded ; whilst the summit of the shell is always acute, and in 

 some species, according to Barrande, septate. 



Hyolithes lanceolatus conforms generally to the generic charac- 

 ters. In its specific features it may be said to be elongately 

 pyramidal and much compressed. It is but little arched longi- 

 tudinally, in fact the shell is almost in one plane ; transversely 

 it is equally little arched, the section being slightly trigonal or 

 unequally oval, the lateral faces being obtusely rounded. The 

 perfect apex of the shell has not come under my notice, nor have 

 I seen any trace of septation. The test is highly ornate, being 

 covered with obtuse concentric rugae parallel to the sectional 

 outline, separated by very shallow interspaces of about their own 

 width apart, both being again traversed by delicate continuous 

 striae following the same direction. The whole of this trans- 



