FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 133 



which is not, or at least very seldom, incurved ; mesial sinus strongly defined, 

 shallow and nearly flat in the bottom ; its sides are straight, which give it a 

 triangular form, in which the sides are about once and one-half as long as the 

 base. Area very large, with sharply angular margins, and greatly inclined 

 forward, and nearly of the same size as the exterior of the valve ; the fissure 

 is high and large, almost equalling in size the mesial sinus. 



Dorsal valve depressed, convex and flattened towards the cardinal extremi- 

 ties, larger than the ventral valve, semi-elliptical in form, with a low but 

 sharply defined mesial fold, which is barely flattened upon the summit. The 

 proportions in height of area, length of ventral and length of dorsal valves, is 

 about as five, six and seven. 



Surface marked by twenty or more simple, rounded or sometimes sub-angu- 

 lar plications on each side of the mesial fold or sinus, the lateral ones of which 

 do not reach the beak, but run out along the margins of the area. In its per- 

 fect condition the shell has been marked by fine concentric striae, traces of 

 which are still preserved, together with stronger imbricating lines of growth. 

 (Hall.) 



This species is associated with Spir. varicosa, and its younger and smaller 

 specimens may sometimes be mistaken for those of that species ; But Spir. 

 segmenta differs from Spir. varicosa in the following points : 



1. In Spir. segmenta the area is always straight and greatly inclines towards 

 the front, which easily distinguishes it from almost any other species of Spiri- 

 fera, except some forms of euruteines. In Spir. varicosa the cardinal area 

 is markedly concave and stands at right-angles to the margins. 



2. Spirifera segmenta has twenty and more plications on each side of the 

 mesial fold and sinus, while Spir. varicosa has only from eight to ten ribs in 

 each half of each valve ; then, again, the plications of segmenta are always 

 smoother than those of varicosa. 



3. In Spir. segmenta the dorsal valve exceeds in size the ventral valve, which 

 is not the case in Spir. varicosa. 



Besides these, there are several other differences which the student of palaBon- 

 tology will soon find, whenever he closely examines numerous specimens of 

 both species. 



The older and larger specimens of this species resemble some Spir. euruteines, 

 but it is easily distinguished from euruteines by the frontward inclination of 

 its area, by its greater transversity, by its smaller size and more numerous 

 plications, and by its dorsal valve exceeding in size the ventral one. All three 

 species are found associated, and the Spir. segmenta forms a kind of connect- 

 ing link between the two other species. 



Prof. Hall states that Spir. segmenta finds its nearest analogue in Spir. 

 angusta of the Hamilton group, and indicates that both species might be 



