316 



PALEOZOIC SYSTEM OF ROCKS. 



The abundance of individuals and the number of species of this 

 order in Silurian times are almost incredible. The accompanying 

 figure srepresent some of the common and characteristic forms. 



It is very difficult to give any general distinctive mark of Silurian 

 Brachiopods, although, of course, the species and even the genera are 

 peculiar, and may be recognized by the paleontologist. It may be said, 

 however, that the straight-hinged or square-shouldered Brachiopods, 

 including the Spirifer family, the Strophomena or Leptena family, and 



the Productus family, are characteristic of 

 the Palaeozoic, though not of the Silurian. 

 Lamellibranchs. — We have said that 

 Lamellibranchs are also found in the Si- 

 lurian, but not so abundantly as the Brach- 

 iopods. Lamellibranchs are divided into 



Fig. 349. 



Fig. 350. 



Fig. 351. 



Fig. 352. 



Figs. 348-352.— Silurian Lamellibranchs : 348. Orthonota parallela. 349. Cardiola interrnpta 

 (after Hall). 350. Avicula Trentonensis (after Hall). 351. Ambonyckia bellistriata (after Hall). 

 352. Tellenomya curta (after Hall). 



Siphonates and Asiphonates, i. e., those with and those without breath- 

 ing-siphons behind. The Siphonates are the higher. At present the 

 Siphonates are the more abundant — in Palaeozoic times the Asipho- 

 nates. We give 

 some figures above 

 (348-352). 



Gasteropods — 

 Vn ivalves.— Land 

 and fresh-iuater 

 Gasteropods have 

 not been found in 

 the Silurian. If we 

 divide marine Gas- 

 teropods or uni- 

 valves into those 

 having beaked 

 FIG - 354 - Fl6 - 355 - shells and those 



Figs. 353-355.— Silurian Gasteropods : 353. Pleurotomaria dryope. . 



354. Pleurotomaria agave. 355. Murchisonia gracilis. having SmOOtll' 



