248 Introduction to A niuial Morphology. 



conical prisms placed at angles of ioor 12, with the surface 

 of the shell. Through the shell are vertical perforations, 

 finer internally than externally, only absent in Spiriferidce 

 and Rhynchonellidae, occupied by processes of the outer layer 

 of the mouth, compared by Huxley to those whereby the 

 tunic adheres to the test in Tunicates. Sometimes the shell 

 is horny and chitinoid (Discina) ; in Lingula it consists 

 chiefly of calcium phosphate (85. per cent.), with carbonate 

 ( 1 2.) and magnesium carbonate (2.). Waldhcimia has more 

 calcium carbonate. The perforations may be branched to- 

 wards the surface (Crania), or fine and parallel, like dentine 

 tubes (Lingula, Discina). The shell prisms are usually equal 

 and with rounded bases, but may be unequal and irregular 

 (Crania). The ventral valve presents, near its articulation, 

 and beneath its beak, a convex or flat surface (hinge arva), in 

 the centre of which is a triangular depression (dcllidium), 

 which may be single (d. discrctuni}, or with a median division 

 (d. sec/tins}, or partly surrounding the perforation (d. ampl< 

 The opening at the summit of the deltidium is the deltidial 

 opening. The shell is often auricle. 1, beside the hin-v area. 

 The two valves may be united only by soft parts (neither by 

 a hinge nor an elastic ligament), or by a long straight line of 

 contact, without teeth (Productidce, Chonetidce), or by a 

 perfect hinge, consisting of two teeth on the ventral valve 

 fitting into two sockets on the dorsal. The outside of the 

 shell is covered by a fine periostracum. The peduncle of 

 attachment varies in length from a few lines to several inches 

 (Lingula) ; it is thick, brownish, semi-cartilaginous, flexible, 

 scarcely elastic, with a single or double (Lingula) horny 

 sheath, with sometimes an inner cylinder of longitudinal 

 muscles within it (inner pillar muscle, as in Lingula). A 

 musculus peduncularis extends from the ventral valve to the 

 base of the pedicle in Waldheimia. A pair of these exist in 

 Rhynchonella. 



Within the shell in Terebratulidoe, Spiriferidae, and 

 Rhynchonellidse are internal processes of calcareous matter, 

 forming a loop for the support of the arms characteristic of 

 the group. These consist of the same material as the shell, 

 but are imperforate in Terebratulidae. They are attached to 



