BRACHIOPODA. 693 



and the two valves may not be similar in this respect. They may be 

 smooth, ribbed, or spinose 1 . One valve may have depressions or sinuses 

 to which correspond elevations or juga on the other. In the hinged 

 Brachiopoda or Testicardines the dorsal valve is furnished with a projecting 

 cardinal process to which are attached the divaricator muscles. At the 

 base of the cardinal process is a right and left socket into which fit the two 

 teeth of the ventral valve. The latter is prolonged posteriorly into a more 

 or less prominent beak which is generally deeply curved and grooved for 

 the passage of the peduncle. The groove is usually converted into a 

 foramen by a ' deltidium/ which consists of two calcareous pieces remaining 

 separate (* d. discretum '), e. g. in Terebratella ; fusing into either a single 

 flat plate ('d. sectans '), e.g. in Terebratula, or one so curved as to embrace 

 the orifice for the peduncle ('d. amplectens'), e. g. in Rhynchonella. In the 

 Spiriferidae, &c. the groove of the beak is frequently closed by a simple 

 calcareous plate or * pseudo-deltidium ' which grows towards the hinge line. 

 The aperture for the peduncle consequently becomes closed, and the shell 

 must then have been set free. The peduncular aperture is wanting in 

 Productus, &c, and the shell is found either free, e. g. Trimerellidae, 

 Strophonemidae, many Productidae^ or attached to some foreign object by 

 the beak of the ventral valve, by its surface, e. g. Crania, Thecidium, or 

 by spines 2 . Discina among E car dines has the ventral valve perforated 

 centrically or excentrically for the peduncle. 



As to the structure of the shell. A thin external periostracum or 

 cuticula covers its external surface and is traceable into a groove at the 

 margin of the mantle (at least in Lingula pyramidata] as well as over the 

 peduncle. The bulk of the shell in Lingididae and Discinidae consists of 

 horny and structureless calcareous layers alternating one with another. A 

 thin layer of horny matter extends to the mantle edge, and is said by 

 Beyer to become continuous in Lingula pyramidata with the supporting 

 substance of the mantle. In Crania the shell substance is almost entirely 

 calcareous, but it is pierced by vertical canals which in the dorsal valve 

 commence with wide apertures, bifurcate three times, and then divide near 

 the surface into a number of minute branches, but in the ventral valve 

 scarcely bifurcate, and are connected with an irregular network of canali- 

 cules. In the Testicardines a fine structureless calcareous layer underlies 

 the cuticula 3 ; it is followed by a layer of calcareous prisms united more or 



1 A living spinose Rhynchonella (R. Doderleinii] has been recently dredged off the coast of 

 Japan. See Davidson, A. N. H. (5), xvii. 1886, who states that spines 'formerly prevailed among 

 Palaeozoic Productidae, Orthidae, &c., and the Oolitic Spiriferidae and Rhynchomllidae] and 

 that 'no spinose Brachiopoda are known from the Cretaceous or Tertiary period.' 



2 The presence and extent of a flattened triangular ' Area ' horizontally and vertically striated, 

 and situated on the dorsal, i. e. inferior aspect of the beak of the ventral valve, and sometimes 

 extending on to the dorsal valve too, is to be noted as a point of classificatory importance. 



* Shipley states that in Argiope the calcareous matter of this layer increases and that the 

 outer ends of the prisms become square and are packed together like bricks. 



