1 4 6 INVERTEBRATE PALAEONTOLOGY 



with Orthonota (? a Taxodont) abounds in the Ludlow 

 series of the Upper Silurian (PI. x. fig. 2). 



Gastropoda are separated into two main sections 

 (Streptoneura and Euthyneura) by the distribution of 

 their chief nerve-trunks ; classification of the next 

 grade is based on qualities of the respiratory organs. 

 Most smaller subdivisions are recognized by the nature 

 of the radular teeth. It is clear, therefore, that inclusion 

 of many fossil Univalves (especially early forms) in such 

 orders and families must be tentative. As far as can be 

 ascertained, both Streptoneura and Euthyneura formed 

 part of the Cambrian fauna. Both sections of the 

 former group (Aspidobranchiata and Ctenobranchiata) 

 seem to have existed in those early times, but Pteropoda 

 are the only Euthyneuran types known. These forms 

 (" sea-moths ") are abundant in recent plankton, but are 

 mostly ill-suited for preservation as fossils. It is possible 

 that the Lower Palaeozoic Pteropods may represent a 

 truly primitive stock, from which not only Gastropods, 

 but even Cephalopods, may have sprung, while modern 

 sea-moths perpetuate the early forms with much re- 

 duction of shell-substance. Hyolithes^ which ranged 

 throughout the Palaeozoic, was sufficiently abundant 

 to be almost a rock-former in parts of the Cambrian. 

 There is little distinctive in its tubular shell,but specimens 

 have been found in Middle Cambrian shales of British 

 Columbia which, by preservation of the soft-tissues, 

 confirm belief in the Pteropod affinities of the genus. 

 Another group of possible Pteropods was that illustrated 

 by Tentaculites (extraordinarily abundant in the Ordo- 

 vician (PI. x. fig. 4), and surviving to the Devonian) and 

 Conularia (Ordovician to Jurassic, least rare in the 

 Silurian). The latter type had a partly septate shell 

 with roughly quadrangular section. 



Aspidobranchiate Streptoneura were essentially Palaeo- 



