Technicalkies of Fossil Shells, 129 



On examining one of the common clam-shells, as they are popularly 

 called, of our rivers, it will be seen that the hard parts of the animal consist 

 ■ef two pieces joined together at the back, where each opens upon the other 

 Hke a door upon its hinges. These shells are concave, so that when closed 

 there is a considerable space within, occupied by the soft parts of the animal. 

 The two shells are called valves, the joint where they are connected together 

 is the hinge, and the small protuberances on the edge of the hinge, the teeth. 

 Similar terms are used in describing fossil shells. 



Fig. 1. Fig. 2. IFlg. 3. 



In the Brachiopoda, such as the Lingula, Orthis, Spirifer, Leptena, 

 Strophomena, and others, there are two valves, and it has been ascertained 

 by the dissection of specimens of those species at present living in the oceans, 

 that one of these valves is placed upon the back and the other on the ventral 

 side of the animal. Hence they are called dorsal and ventral valves. Fie, 

 1 shews the ventral valve of Orthis tricenaria, a Trenton Limestone sj^ecies, 

 very abundant in that rock at the lower end of the Allumette Island on the 

 Ottawa. Fig. 2 is a dorsal view of the same specimen ; in this figure it will 

 be observed that the dorsal valve is shorter than the other. It extends only 

 to the straight line across the figure near the top. The broad triangular 

 space above the line is a portion of the ventral valve. Fig. 3 is a side view 

 of a specimen shewing how the ventral valve projects above the other in a 

 sharp hook-like termination, which is called the beak. Both valves have a 

 beak, but that of the ventral is almost always the largest, and projects the 

 highest. The hinge line is simply the hinge portion of the shells. 



Fig. 4. Fig. 5. 



Fig. 4 is an end view of Orthis tricenaria looking at the hinge, the 

 ventral valve being uppermost. The valves are each terminated by a flat 

 space, as if they had been cut off with a knife. These constitute what is 

 called the " cardinal areci)' "■ hinge area,^ or simply " the area.'' The area 

 in the genus Orthis is penetrated in the centre by an angular aperture, shewn 

 in fig. 4, by the lozenge-shaped black space in the centre. I.Tirough this 

 aperture it is supposed the pedicle passed, by which the animal was attached 

 to the bottom of the sea. It is called the ''foramen,'' or "fissure." It 

 consists of a triangular notch in each valve, deepest in the ventral valvCi 



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