SILURIAN ANIMALS 



Fig. 335. Fig. 336. Fig. 337. 



Figs. 333-337.— Silurian Crinoids and Asteroids: 333. Mariacrinus nobilissimus (after Hall). 

 334. Homocrinus scoparius (after Hall). 335. Heterocrine simplex (after Meek). 336. Protas- 

 ter Sedgwickii. 337. Palaeaster Shsefferi (after Hall). 



Mollusks — Acephals or Bivalves. — Bivalves may be divided into two 

 great sub-classes, viz., Lamellihranchs (leaf-gills) and Brachiopods (arm- 

 feet). The valves of Lamellibranchs are right and left; those of 

 Brachiopods are upper and lower, or dorsal and ventral. Brachiopods 

 are much less highly organized than the other sub-class, and differ so 

 essentially in their organization that some of the best naturalists re- 

 move them not only from the class of Acephals, but from the depart- 

 ment of Mollusca, and ally them rather with the Worms. Their gen- 

 eral resemblance in external form to bivalves makes it more convenient 

 to treat them under that head, until the question of their affinity is 

 more definitely settled. Brachiopods are very abundant in the Silurian. 



