136 BRITISH CARBONIFEROUS BRACHIOPODA. 



been hitherto recorded ; but a few forms of those sub-genera occur in equivalent rocks of 

 other countries, such as Belgium and India. In all British species of Productus and 

 Chonetes, the shell is more or less concavo-convex, oval, semi-oval, or angular, and generally 

 auriculated ; the hinge-line is straight and strong, with or without teeth and sockets for 

 the articulation of the valves. All well-authenticated species of Productus, hitherto 

 examined, have shown themselves to be edentulous, but whether this character was general 

 and without exception may remain a question for further consideration ; anyhow, the 

 dorsal valve must have turned on its long hinge-line with as much precision as in Chonetes, 

 which possessed regularly articulating teeth. It has been often asserted and believed that 

 Productus might be distinguished from its sub-genera by the total absence of an area; and 

 although this is the prevalent character of the genus, still in certain species, such as 

 P. sinuatus, a perfectly developed area is generally present in the ventral valve. There 

 exists also an occasional tendency to the formation of hinge-area in several species, as may 

 be seen, for example, in the remarkable example of P. semireticulatus of which a 

 representation is given in PI. XLIII, fig. 5. 



All species of C/ionetes at present known have, in addition to the regular articulation, 

 an area in each valve, this being larger in the ventral than in the dorsal one, which is also 

 divided by a fissure, more or less arched over by a pseudo-deltidium, the cardinal process 

 of the opposite valve filling up and effectually closing any portion that might have other- 

 wise remained uncovered. 



The external surface in Productus varies according to the species ; in some, it is almost 

 smooth, in others longitudinally and finely striated or coarsely costated, as well as inter- 

 sected by numerous concentric wrinkles or lines of growth. All the species appear to have 

 been furnished with tubular spines ; in some forms they are small, delicate, and so closely 

 packed as to conceal every portion of the shell, with the exception of the area ; while in 

 others they are irregularly scattered, and chiefly confined to the auriculate portions of the 

 valves. In certain species, the spines exceeded by four or five times the length of the 

 shell ; and while some were almost as delicate as the hair of one's head, others exceeded a 

 line diameter ; the dimensions of the shell having nothing to do with that of the spine, 

 for in some small species these were few and large, while the reverse has occasionally been 

 found to be the case with species of the largest dimensions. 



Chonetes differs also somewhat from Productus by the manner in which its spines are 

 disposed along the cardinal edge, these last sloping outwardly, and increasing in length as 

 they approach the extremities of the hinge-line ; but in many species, in addition to these, 

 there existed on the surface of the valves small spines, disposed as in Productus. 



The intimate shell-structure has been described by Dr. Carpenter to be perforated, 

 and that, where the shell is furnished with spines, the perforations are continued into 

 them, and that such passages are of more than average dimensions. In Productus the 

 internal surface of the dorsal valve is more or less convex, and presents in the middle of 



