25 
longitudinal septa, separated from one another by a slight space, and whicli 
support two oblique plates corresponding to the plates placed in a V-shape in 
the opposite valve. 
Horizon and Localities. — This species is very abundant in the Silurian 
beds of Llandovery, and there sometimes forms entire deposits to the exclusion 
of all other fossils. It is found in England, Ireland, Scotland, America, and 
Eussia. In Australia it is found in a brownish shale, containing innumerable 
internal and external casts of Atrypa reticularis, Linne, which has been 
collected at Duntroon. One of the specimens contains, besides tlie two species 
I have mentioned, specimens of liemisplmrica. Sow., and of Strojjhomenes 
compressa, Sow., species that are selected by Sir Ptoderick Murchison as being 
the most characteristic of those occurring in the upper beds of the Llandovery 
(Llandovery Locks). There cannot then be the least doubt as to the 
geological horizon of the rock that has yielded these specimens. 
Genus — OP^THIS, Lalman. 
3. Orthis CANALICULATA, Lindstrom. 
Ortllis orbicularis, Sclimidt, 1859, Beitr.ag zur geologic der Insel Gothland, p. 44! (non 
Sovrerby) . 
„ canaliculata, Lindstrom, 1860, Ofvers. K. Vet. Akad. Torhandl., Iso. 8, p. 268, 
pi. 16, fig. 10. 
,, ,, Davidson, 1871, Mon. Brit. Sil. Brach., p. 218, pi. 27, fig. 12-13. 
Shell transversely oval, widest towards the middle ; hinge line much 
shorter than the transverse diameter of the valves ; cardinal angles rounded, 
front slightly sinuated, ventral valve moderately convex, slightly depressed 
laterally; the area is triangular and hollowed like a canal; dorsal valve 
much less convex than the ventral, it would appear almost flat were there 
not in the median portion a fairly well marked sinus which extends from the 
beak to the margin, and divides it into two equal parts. The surface of 
both valves is covered by a great number of small ribs increasing either 
by bifurcation or by interposition, and crossed, particularly towards the 
margin by concentric growth striae, slightly regular, but easily seen by the 
naked eye. 
Dimensions.— eleven millimetres ; breadth, fifteen millimetres; 
breadth, fifteen millimetres ; and thickness, four millimetres. 
