24 PALEONTOLOGY OF KENTUCKY. 



Plate I, figure 25. Among the fossil shells of even the older Palaeozoic strata, 

 we find sometimes Brachiopod shells so well preserved that even the valves 

 may be opened as far as the teeth and sockets of the hinge will allow. Most 

 of the shells of Brachiopods have a peculiar structure, consisting of flattened 

 prisms of considerable length, arranged parallel to each other with great regu- 

 larity, and obliquely to the surfaces of the shell, which is also perforated by 

 canals (see Plate I, figure 22). This great class of Mollusca has derived its 

 name from the two long ciliated arms developed from the sides of the mouth, 

 with which each animal is provided as means to create currents which bring 

 to it its food. These arms (Plate I, figure 9) were considered by former natu 

 ralists as instruments of locomotion ; but it is now ascertained that they, out- 

 side of their first mentioned purpose, serve mainly as breathing organs ; the 

 erroneous name Brachiopoda (arm-footed), should, therefore, be corrected into 

 Brachionabranchia (arm-breathers). Most of the Brachiopoda are provided 

 with an internal skeleton, consisting of two spiral processes in the Spiriferirhe 

 (see Plate I, figures 7 and 8), whilst in others, the Terebratula and Thecidin, 

 this skeleton takes the form of a loop, as seen in Plate I, figures 6 and 26. 

 This skeleton serves as a support for the brachial membrane. The prominent 

 parts of the Brachiopod shell are the following : Dorsal valve, the upper one, 

 which is usually the smaller, ventral valve, the lower one, which is generally 

 the larger. The beak is that portion of the valve which terminates above the 

 hinge -line in a sharp point or in a perforation (as seen in Plate I, figure 25 

 "a 5 '). This perforation is called the foramen. The most convex portion of 

 the valve near the beak is known as the umbo. Most of the shells have below 

 the beak a triangular opening, which, in valves without hinge line, is closed 

 by a separate body, consisting of either one or two pieces, the deltidium. In 

 shells with a hinge area, as the Spirifera and Cyrtina, said area is divided in 

 two by the triangular opening, which is partly or entirely closed by the growth 

 of the shell. This is known as the pseudo- deltidium. In some shells the 

 outline of the valve forms on both sides of the hinge a straight line, the hinge 

 line, which may be as long as the greatest breadth of the shell, or may be so 

 short as scarcely to be noticeable. The area which sometimes exists between 

 hinge-line and beak is the hinge -area, best developed in the shells of Cyrtina. 

 The hinge-area may be straight or curved, and is always divided by the pseudo - 

 deltidium. 



The valves of a shell are either both convex, or one convex and the other 

 plane, or one convex and the other concave. In some shells, as in Spirifera, 

 the ventral valve has a strong depression extending and enlarging from the 

 beak to the front, dividing the valve into two equal halves. This depression is 

 known as the mesial sinus. Corresponding with this mesial sinus is an eleva- 

 tion in the dorsal valve, extending and enlarging also from beak to front, and 



