Lower Silurian Fossils of Canada, 205 



of the species, and some others where it is difficult to decide their true relations 

 when we have not a scries shewing their gradations." 



Palaeontology of New York, vol 1, page 102. 



Professor McCoy in describing the English specimens, says : — " This 

 species is longitudinally, semi-elliptical, or transversely subquadrate ; 

 receiving (ventral) valve gently convex along the middle, with a very short 

 mesial sulcus close to the beak, gently deflected at the side margins when 

 old; entering (dorsal) valve flat, or slightly concave, cardinal area very 

 narrow, inclined back at about 120°, surface of both valves radiated with 

 very fine linear thi-ead-like striae, separated by equally wide, flat, coarsely 

 punctured spaces, when partially decorticated; stria3 thicker and closer 

 together, when the surface is preserved ; stria3 of two, more or less distinct 

 sizes, usually three of the finer, between each pair of the larger ; the middle 

 subsequently increasing in size, and a new pair of small striee being mterca- 

 lated on each side ; about eighteen stri^ in the space of two lines, at six 

 lines from the beak ; interior of the valves marked with radiating external 

 striae, and with f ery numerous obtuse granules, nearly corresponding with 



the external punctures." 



Sedgewick and McCoy's Palaeozoic Rocks of Great Britain, page 233. 



This species is found in the Trenton Limestone abundantly, and also in 

 the Hudson River group. The specific name is from the Latin Alternates, 

 meaning alternated or changed by turns, in allusion to the difi'erent sizes of 

 the striae. It was originally placed in the genus strophomena by the 

 American geologists. Afterwards called a lepteim, both in Europe and 

 America, but since the recent more extended observations upon the internal 

 structure of these fossils, the genus Strophomena, as mentioned in a previous 

 article, has been restored. The terms receiving and entering valves, made 

 use of by Professor McCoy, are the equivalents of ventral and dorsal 

 valves, as now more generally used. When the Paleontology of Xew 

 York was written, these latter two terms were reversed in their meaning, 

 which accounts for the brackets in the above quotations. 



Leptena deltoidea is another fossil very like S. Alternata above figured. 

 Its principal differences consist in its being usually more convex and 

 pointed in front, like Fig. 4, than rounded below, like Fig. 3. There are, 

 also, in most good specimens, a number of concentric wrinkles on the 

 surface of the shell. We have not yet ascertained whether this fossil should 

 be called a Lejotena or a Strophomena. It occurs in the Trenton Limestone 

 in many localities. The specific name is founded upon the form of the 

 fossil, approaching the triangular shape of the Greek letter delta. 



Orthis pectinella has the ventral valve flat towards the margin, and 

 the dorsal valve convex, most prominent in the centre. The ventral valve 

 has a low area, which is almost at right angles with the plane of the shell. 

 The surface is marked by from 22 to 30 prominent, rounded, radiating ribs, 

 with wide spaces crossed by small elevated concentric lines. This shelly 

 when perfect, so that the ribs and concentric lines are well shewn, is a very 

 pretty and neat fossil. There are several others, which are either varieties 



