54 Guide to the Invertelrata. 



frAlLEBT Terehratula, a hinge is developed at the hinder end of the shell 

 East Side (I^'agram 4). These have therefore been called Articulata or 

 Wjftll-casea Testicardines.^ As classificatory divisions, however, these are not 

 ^^w ^^* altogether satisfactory, for the Articulata are necessarily derived 

 eaaea horn the Inarticulata, and intermediate forms are not rare. 



§a-«8. In the relations of the peduncle to the valves one can trace a 



gradual evolution. The simplest type is seen in Paterina (from the 

 Cambrian, Fig. 96) and Lingula (Cambrian to present day, Fig. 100), 



Fig. 96. — Paterina Labradorica, Billings, sp. Cambrian, 

 An example of the Atremata^ aud the simplest known form of BracMopod shell. 



where the peduncle simply passes out betvreen the valves and is not 

 enclosed by either of them ; there is therefore no hole, or trema 

 irprjiitq)^ through vt^hich the. peduncle may pass, and such genera 

 constitute the order Atremata. In Trematis, Discina (Fig. 97), 

 tSiphonatreta, and their relatives (mostly Ordovician and Silurian), the 

 peduncle is restricted to the ventral valve ; it lies in a groove or 



Fia. 97. — Discina ciree, Billings. Ordovician. 

 An example of the Neotremata. 



The peduncular valve, showing the delthyrium, which has become surrounded by 

 the valve, and is also partly filled in from underneath by a shelly deposit. 



fissure (" delthyrium "), which remains open in primitive forms, 

 but closes round the peduncle (forming a rprj/^a) in later forms : such 

 genera constitute the order Neotremata. Kext, the fissure or 

 "delthyrium" in which the peduncle lies, assumes a triangular 

 shape ; the peduncle lies towards the apex of the triangle, and 

 itself secretes a single shelly plate ("pseudo-deltidium "), which 



^ Testa, a shell ; caido, a hinge. 



