FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 97 



Cyrtina hamiltoniae, var. recta, HALL. 



Plate XIII., figures 13 to 16. 

 Oyriina hamiltonensis, var. recta, Hall. Pal. N. Y., Vol. IV., p. 2701867. 



Prof. Hall separates this shell from the species Hamiltoniae, and makes a 

 variety of it, which is based upon two points : first, upon the plane, flat area 

 and straight beak, and second, upon the angular plications. If only the ex- 

 treme forms were in existence, those points would suffice to maintain the new 

 variety ; but inasmuch as the species Cyrtina hamiltoniae is very variable in 

 shape, we find individuals which gradually lead from one form to the other. 

 Three groups may be distinguished among these shells ; in the first we find 

 specimens with very arcuate or concave cardinal area, such as shown in figure 

 11, plate 13 ; in the second group the cardinal area is only slightly curved near 

 the beak, and the latter is always somewhat; twisted or turned to one side ; and 

 in the last group we find the forms, separated by Prof. Hall from the main 

 species, and put in the variety recta. In these shells the cardinal area is 

 always plane or flat, and leaning generally towards the front, and the beak is 

 without any curvature. Intermediate forms connect the first with the second 

 group, and others lead from the second to the third group. 



Formation and Locality. Found associated with the preceding species. 



Genus Meristella. Han. 



Meristella, Hall. 13th Regent's Report 1860. 

 Etymology : a diminutive of Merista. 



The shells of this genus are oval, ovoid or sub-orbicular, elongate or rarely 

 transverse ; valves unequally convex, with or without a median fold or sinus ; 

 beak of the ventral valve often with a circular foramen and incurved over the 

 umbo of the dorsal valve. Area none ; valves articulating by teeth and 

 sockets ; surface smooth or with fine concentric lines of growth, and with 

 very fine, indistinct or obsolete radiating striae. 



The interior of the dorsal valve is marked by the presence of a strong 

 hinge-plate or cardinal process, and from the base of this proceeds a thin, 

 longitudinal septum, which often extends for half the length of the valve. 

 The interior of the ventral valve shows a triangular fissure below the beak, 

 which joins a semi-circular perforation at the apex. At the base of this fissure 

 are two strong teeth, which extend in thickened or slender plates to bottom of 

 cavity, and curve around upper part of the muscular area, which is broadly 

 triangular or ovate. There is sometimes a thickening of the shell at the base 

 of the rostral cavity, which abruptly limits the muscular impression ; but there 

 is neither septum nor rudiment of one as in Merista, 



QEOL. SUK. 13 



