256 



MOLLUSCA. SCAPHOPODA 



Hyolithes ( = Theca) (fig. 101). Shell calcareous, straight, 

 rarely curved, pyramidal, its section 

 triangular, elliptical, semi-elliptical 

 or nearly circular ; surface smooth 

 or striated ; posterior part some- 

 times crossed by septa. Aperture 

 with an operculum. Cambrian to 

 Permian. Ex. H. elegans, Ordovi- 

 cian. 



Conularia. Shell thin, formed 

 of chitin, more or less impregnated Fig. 101. Hyolithes from the 

 with lime ; generally straight, pyra- ^jambi -ian showing the oper- 

 midal, with four sides ; each angle 



of the pyramid with a straight groove ; each lateral face may have 

 a median longitudinal groove. Apical part of shell sometimes with 

 a few convex septa. Surface smooth, or ornamented with numerous 

 transverse, parallel, angulated ridges, and sometimes with longitu- 

 dinal ridges. Aperture partly closed by incurved triangular lobes. 

 Ordovician to Lias. Ex. C. quadrisulcata, Carboniferous. 



Tentaculites. Shell calcareous, thick, solid, in the form of 

 a greatly elongated cone, straight or slightly curved, with circular 

 section ; apical part with septa, its end often with a vesicular 

 enlargement. Surface provided with prominent, transverse, parallel 

 rings, and with transverse and longitudinal striae. Ordovician to 

 Devonian. Ex. T. anglicus, Bala Beds. 



Other genera, which appear to be allied to the preceding, are 

 Hyolithellus, Salterella, and Coleolus from the Cambrian. 



CLASS III. SCAPHOPODA 



The Scaphopoda include only a few genera, which in 

 some respects resemble the lamellibranchs, and in others 

 the gasteropods. The body is elongated, and bilaterally 

 symmetrical. The mantle is cylindrical and open at both 

 ends ; it secretes a straight or slightly curved tubular 

 shell which is also open at both ends. The foot is 



