PROSOBRANCHIATA. 



763 



Lindstrom, the muscular scar is in the form of six pairs of disconnected 

 impressions, arranged in an oblong circle which is open in front. The 

 species of this genus are principally Ordovician and Silurian. Metop- 

 toma, as now restricted, differs from the preceding in having the anterior 

 side broadly truncated (fig. 645, f), and in the fact that the muscular scar 

 is in the form of a continuous horse-shoe. The genus appears to range 

 from the Ordovician to the Carboniferous, but as the interior markings 

 of many species are unknown, it is impossible at present to sharply 

 separate this type from Tryblidium. 



Family 2. Fissurellid/e. — In this family the animal has the 

 gills symmetrically disposed on the two sides of the body. The 

 shell is conical and limpet-shaped, with a notch in the anterior 



'%!!%' 



Fig. 645. — Fissurellidae, Capulidae, and Patellidae. a, Fissurella labiata — Eocene ; b, Rimida 

 Blainvillei — Eocene; c, Emarginula Gtierangeri — Cretaceous; D, Hipponyx cornucopia — 

 Eocene ; E, Crefiidula c^fo/a— Miocene ; f, Metoptoma imbricates.— Carboniferous ; g, Patella 

 costaria — Eocene. 



margin, or a perforation at or near the apex, corresponding with the 

 anus (fig. 645, a and c). The muscular impression is horse-shoe- 

 shaped and is open in front. 



The genus Fissurella (fig. 645, a) comprises the so-called "Key- 

 hole Limpets," distinguished by having the apex of the shell perfo- 

 rated by a larger or smaller, generally oval aperture. Very dubious 

 examples of the genus have been described from the Devonian and 

 Carboniferous rocks, but the earliest undoubted types appear in the 

 Jurassic rocks, from which time the genus has continued to the 

 present day. In the genus Rimula (fig. 645, b), ranging from the 

 Lias to the present day, the perforation, instead of being at the apex 

 of the shell, is placed a little above the anterior margin. Lastly, in 

 Emarginula (fig. 645, c) the anterior margin is furnished with a 

 longitudinal notch or slit. The oldest species of this genus has 



