LOWER SILURIAN. 



173 



Among the Acadian fossils, no remains of Crinoids have yet been 

 found. The Brachiopods include species of Lingulella (Fig. 248), 



is more circular, and the shells have a hinge-area, and a regular hinge with teeth; 

 it also differs in being attached by the beak of the ventral valve. Aulosteges is also 

 similar to Productus in general form and spines; but there is a broad triangular hinge- 

 area, and the beak is twisted somewhat to one side. 



Figs. 238-246. 



Fig. 238, Productus aculeatus, dorsal view ; 239, Productus semireticulatus, ventral view ; 239 a, 

 section of Productus, showiog the curvature of the valves ; 240, Chonetes lata, opposite views ; 

 241, Calceola sandalina ; 242, Crania antiqua ; 243, Discina lamellosa, side-view ; 244, id. showing 

 foramen ; 245 a, b Siphonotreta unguicnlata, opposite views ; 246 a, b, Obolus Appollinis. 



Koninckina is like Productus in form, but has about the same relation to the Produc- 

 tus family as Atrypa to the Rhynchonella family. 



6. Discina Family. —In Discina (Figs. 243, 244) the form is orbicular or oval, and the 

 valves low-conical ; there is a slit through the ventral valve, beginning at or near the 

 highest point. The genus Orbicula is here included. Trematis is similar; but one 

 valve has the umbo or prominent point marginal, or the slit reaches nearly to the 

 margin. In Siphonotreta (Fig. 245),the form is ovate; the beak projects at the margin, 

 is somewhat pointed, and has a small aperture. Acrotreta has the perforate valve 

 elevated into a high oblique cone. 



7. Lingula Family. — Lingula (Fig. 151) is narrower than high, and pointed at the 

 beak; valves equal, thin. Obolus (Fig. 246) is rotund or rotund-ovate ; valves a little 

 unequal, the dorsal valve being the smaller and least convex, as in most Brachiopods ; 

 muscular impressions, six, — two median, two lateral, and two very near the umbos 

 (Fig. 246 b), — having some approximation to the Cranice. Obolella of Billings has still 

 different muscular impressions, as shown in Fig. 273. 



8. Crania Family. — The genus Crania has internal markings as in Fig. 242; and the 

 shell was attached when living,by the substance of one valve to a rock or other support. 



9. Thecidium Family. — Thecidium contains thick-shelled species, higher than wide, 

 having a pointed beak, very large triangular hinge-area, and internally digitate mus- 

 cular impressions; commenced in the Trias, and has a single living species. 



Davidsonia is a genus of rare occurrence and undetermined relations. There is some 

 resemblance to Leptcena ; but it has a pair of low and faint spiral cones on the inner 

 surface of the larger valve. 



The following genera have species in the existing seas; and those having an asterisk 

 are known only as recent. In the Terebratula family, the genera Terebratula, Wald- 



