184 PALAEOZOIC TIME LOWER SILURIAN. 



Trilobites were the largest animals of the seas, and the highest in 

 rank. There were numerous kinds, and they varied in length from 

 a sixth of an inch to two feet. Fig. 245 is a diminished representa- 

 tion of one of them : it is a species of Paradoxides (P. Harlani), from 

 the Primordial rocks of Braintree, near Boston. 



valves in general nearly equally convex ; the hinge-line usually not long, with a 

 small cardinal area; a few species resemble a narrow Spirifer, and have a medial 

 fold and long hinge-line. Orthisina has the hinge-area very large and reversed- 

 triangular, with a convex deltidium, and the shell subquadrate. Strophomena 

 contains thin D-shaped species (figs. 219-221), with a straight hinge-line 

 about as long as the width of the shell, a very narrow hinge-area, the dorsal 

 valve often very concave, with the ventral bending to correspond, and the four 

 adductor muscular impressions in the same transverse line. Leptsena is similar 

 (figs. 222-224), but has the four muscular impressions of different character, 

 as seen in fig. 223, while in Strophomena they are as in fig. 220. 



5. Proditctus Family. — In the genus Productus (figs. 228, 229) the beak is 

 very full; hinge-line usually a little shorter than the width of shell; no 

 true hinge-area, and no beak-aperture ; the smaller valve concave ; the eurface 

 of the shell spinous, the spines tubular. The margin of the shell is prolonged 

 downward often to a great length, and sometimes closes around into a tube. 

 Ghonetes (fig. 230) has a straight hinge-line, commonly as long as the width of 

 the shell, the form rather thin, with the beaks not full and prominent, resembling 

 Leptaena; smaller valve concave; hinge-edge of larger valve furnished with a 

 few spines. Strophalosia is much like Productus in form and spines, but is more 

 circular, and the shell8 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. 



6. Discina Family. — In Discina (figs. 233, 234) 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. 235) 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. 161) is narrower than high, and pointed at 

 the beak; valves equal, thin. Obolus (fig. 236) 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 medial, two lateral, and two very 

 near the umbos (fig. 236 b), — having some approximation to the Cranise. Obolellct 

 Billings has still different muscular impressions, as shown in fig. 244 A. 



8. Crania Family. — The genus Crania has internal markings, as in fig. 232, and 

 the shell was attached when living by the substance of one valve to a rock or 

 other support. Calceola (of doubtful relations) is like a cone flattened on one 

 side and closed with a lid, as in fig. 231 ; valves not hinged. 



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



