204 



Lower Silurian Fossils of Canada, 



we have seen, the ridges in this cavity are not so conspicuous as they arc in 

 this figure, which is taken from Hall's Palseontology. The length of the 

 hinore line or the straigrht side of this fossil is from an inch and a half to two 

 inches, while the height from beak to base is from one inch to an inch and a 

 half. The specific name appears to be from the Latin, ^Ivm^ a thread, and 

 tcxtum, woven, in allusion to the woven appearance of the striae on the shell. 

 This character is not always clearly seen. This species occurs only in the 

 Trenton Limestone, and is most numerous at the base of the formation — 

 rarely in the upper part. For the technical terms used in this article, (see 

 articles 18 and 19.) 



Fisr. 3. 



Fia:. 4. 



Figs. 3 and 4. — Strophomena alternata, (Conrad.) 



This species is one of the most common and frequently quoted of the 

 brachiopoda of the Lower Silurian rocks, and it is one of the most variable 

 in form. It is found both in England and America, and we shall therefore 

 give the descriptions of the pateontologists on both sides of the water, in the 

 hope that from the two the species may be recognized v/ith the more 

 certainty. 



Professor Hall says that this shell is '^ broadly semi-oval ; length and 

 breadth about as 12 to 15 ; hinge line, in perfect specimens, a little longer 

 tliau the width of the shell, slightly reflected at the extremities, which some- 

 times become short acute ears ; cardinal area narrow, the callosity of the 

 ventral valve, nearly filling the triangular foramen of the dorsal 

 valve ; beak uniformly perforated with a minute circular opening ; 

 dorsal valve, depressed convex, sometimes more convex in the middle, 

 suddenly deflected near the margin, and flattened towards the cardinal 

 line ; ventral valve concave, gradually or sometimes suddenly inflected 

 towards the basal margin ; surface marked by five rounded radiating striae, 

 which alternate at unequal intervals with coarser ones ; striae increasing in 

 numbers towards the margin of the shell, crossed by fine elevated concentric 

 lines and a few imbricating lines of growth. 



*•■ In this species the striae are usually of two sizes, the coarser and more 

 elevated ones having from four to six finer ones between them, the latter 

 increase in number as they recede from the apex, and one of them in the 

 centre of the fascicle becomes enlarged, and rises above the others. Some of 

 the specimens figured are strongly marked individuals where these chf^racters 

 are very distinctly preserved ; but there are many variations from the typo 



