Fossils of the Lower Silurian RocJcs of Canada. 41 



Orihis testudinaria with those of New York shews no essential difference ; 

 the former being a httle more elongated, and the dorsal valve more convex 

 than in the prevailing forms of the Trenton Limestone." In England it is 

 found in the Llandeilo and Caradoc formations of the Lower Silurian. In 

 Canada it ranges from the Black Kiver Limestone upwards, through the 

 Trenton Limestone, Utica Slate, and Hudson River group. In the Utica 

 Slate it is rare, and most abundant in the Trenton Limestone. 



The generic name Orihis is from the Greek Orthos, " straight," in 

 allusion to the straight hinge line. The specific name testudinaria is from 

 the Latin testudo, a '' tortoise," this species having a fanciful resemblance 

 to a tortoise. In the earlier works of the American Geologists, this fossil is 

 called Orthis striatula, and it is also so named in Sir Roderick Murchison's 

 new work, Siluria. It thus appears that there yet remains some difference 

 of opinion as to the correct appellation of the species. 



Leptena sericia. — All the species of the genus Leptena have a 

 straight hinge line, and consist of two thin valves, one of which is convex, 

 or rounded, and the other either flat or concave. The small engraving, at 

 the right of Fig. 2, is a section through a specimen of L. sericia from 

 the beak to the base, and shows how one valve is bent and fits into the 

 corresponding outward curve of the other. 



This species is very broad and straight along the hinge line ; its width 

 being usually more than twice its length. The ventral valve is convex ; the 

 dorsal concave, and the surface is marked by fine striae, which are even and 

 uniform, or alternating with stronger ones ; striae increasing in numbers 

 towards the margin, granulate or papillose ; crossed by a few lines of 

 growth ; surface shining." " This beautiful and abundant little shell is 

 readily distinguished by its almost perfectly semi-oval form, with fine 

 papillose striae, alternating with stronger ones ; the latter are often obsolete, 

 and the surface appears uniformly striated. 



Very abundant in the Trenton limestone ; — Hudson river group, and 

 more rare in the Clinton group. Speaking of this and other species. Sir R. 

 Murchison says, " of the two species of Leptgena which are prevalent in the 

 lower division, the most frequent is L. sericia ; w^hich occuiTing in 

 swarms among the slates of Snowdou, is also frequent in the Caradoc 

 Sandstone of Shropshire and of the Malvern Hills ; whilst the L. tranver- 

 salis, published originally as a fossil of the Wenlock shale, is now found in 

 Llandeilo formation of Wales and Westmoreland. The former of the two 

 last mentioned species has indeed an universal range ; being known in 

 Russia, Scandinavia, Central Germany, the British Isles and America. 



Leptena is from the Greek, Leptos, thin ; Sericia, Latin, silken, io 

 allusion to the shining or silken exterior of the shell. 



