PTEROPODA. 



807' 



tion in the fossil state, no record of the past existence of the genus 

 can be obtained. 



Family 4. Hyolithid^e. — With this family we enter upon the 

 consideration of a series of remarkable Palaeozoic fossils, which 

 have been generally referred to the Pteropoda, though their true 

 affinities cannot be considered as certainly established. The type 

 of this family is the genus Hyolithes ( = Theca and Pugiunculus), 

 which has been generally regarded as allied to the recent Clio or 

 Styliola (fig. 722, c), though the dimensions of the shell much 

 exceed those of the ordinary species of the latter, and there are 

 other differences as well. The shell (fig. 722, d, f, h) in Hyolithes 

 is bayonet-shaped or conical, usually straight, but sometimes curved, 



Fig. 722. — A, Pterotheca corrugata — Ordovician (after Salter); b, Pterotheca transversa — 

 Ordovician (after Salter); c, Styliola (Clio) depressa — Miocene; d, HyolitJies (Theca) oper- 

 culatus; and E, its operculum— Upper Cambrian (after Salter); f, Hyolithes aratus — Upper 

 Cambrian ; and G, Cross-section of the same (after Salter) ; h, Hyolithes acutus — Silurian (after 

 Eichwald). 



of thin texture, transversely striated or smooth, sometimes with 

 marginal ribs, but without lateral appendages. The mouth of the 

 shell is trigonal, and in some forms, at any rate, is provided with 

 an operculum (fig. 722, e), or occasionally furnished with curved 

 lateral appendages. The length of the shell varies, but is com- 

 monly from an inch to an inch and a half. The species of 

 Hyolithes are most abundant in the Upper Cambrian and Ordovi- 

 cian rocks ; but there are various Silurian and Devonian forms, and 

 the genus is recorded as occurring in both the Carboniferous and 

 the Permian rocks. Pterotheca (fig. 722, a and b), of the Ordo- 

 vician, in many respects resembles Hyolithes; but the median 

 dagger-shaped shell is bordered by lateral concentrically-striated 

 expansions or alations, thus coming to superficially resemble the 

 carapace of certain of the Phyllopods. 



The Devonian genus Coleoprion has a cylindrical and conical 



