342 PALAEONTOLOGY OF NEW-YORK. 



The species passes through a variety of forms, mostly induced by age, and the extremes might 

 easily be mistaken for distinct species. In the young specimens, the depth of the valves is 

 much less than in older ones ; the beak of the dorsal valve is also much more elongated and 

 acute, and less incurved than in older specimens, where it is obtuse and much incurved. The 

 shell, particularly towards tlie beaks, becomes thickened by age, and the imperfect specimens 

 often show numerous lamella;. The interior plates, particularly of the dorsal valve, have the 

 distinct lamina?, of which they are composed, separated by weathering. 



The larger specimens of this species approach in form to Pcntamertts knighiii ; but the ribs 

 are smaller, and the ventral valve more elongated. 



Fig. 1 a. Cast of the dorsal valve of a. young specimen, preserving the shell towards the base. 



Fig. 1 b. Interior of the preceding, showing the elongated beak and foramen. 



Fig. 1 c. Profde view of a young specimen, the shell being nearly all removed. 



Fig. 1 d. Dorsal valve of the preceding specimen. 



Fig. 1 e. Cast of the dorsal valve of a small specimen, showing the original thickness of the shell 

 at the beak from the extension of the central plate. 



Fig. 1 f. Cast of the dorsal valve of a larger specimen. 



Fig. 1 g Sc h. Ventral valves having the shell partially preserved. 



Fig. 1 «. Profile view of a ventricose specimen, the beak of the dorsal valve being reduced by 

 abrasion. 



Fig. 1 /i. Cardinal view of the largest individual seen, showing the numerous laminae of which 

 the thick shell is composed at the beak. The foramen is incorrectly represented in 

 the figure : it should e.xtend beneath the beak of the dorsal valve. 



Fig. 1 /. Profile view of the same. 



Fig. 1 m. Cast of the dorsal valve, having the beaks little extended. The space between the cast 

 and the surrounding stone indicates the thickness of the shell, and the central plate 

 is shown to be composed of several (at least six) distinct lamina;. 



Fig. 1 o. A cast of the dorsal valve, the two sides separated more than usual. 



Fig. 1 p. The interior of the dorsal valve, where the outer laminae of the central plate have be- 

 come separated by the removal of the inner ones, giving the appearance of two 

 plates. 



Fig. 1 r. The interior of a dorsal valve, showing the separated lamina; of the internal plate near 

 ihe beak. 



Fig. 1 s. Cardinal view of an imperfect specimen, the ventral valve showing the two plates e.x- 

 tending from near the beak downwards. 



Fig. 2. The interior of a small dorsal valve, perhaps a distinct species. 



Position and locality. In the light colored limestone at Gait, Canada West. And in the 

 limestone on Lime island, St. Mary's river ; Point St. Vital, Lake Huron ; at numerous points 

 on the north shore of Lake Michigan, and on the peninsula between Green Bay and Lake 

 Michigan. 



