164 BRITISH CARBONIFEROUS BRACHIOPODA. 



illustrated by Messrs. de Verncuil and de Koninck, who state that, although the tube was 

 generally simple, it sometimes bifurcated, and then two independent tubes were pro- 

 duced. Its direction is generally that of the longitudinal axis of the shell, but it is often 

 irregularly twisted to the one or the other side, and may likewise remain straight for 

 some time before suddenly bending and assuming another direction, as represented in one 

 of the specimens figured in my plate. The tube has also attained at times considerable 

 length, one Belgian example figured by Prof, de Koninck being nearly two inches in 

 length, with not more than four lines in width, and although the specimen was broken 

 and imperfect at its extremity, the width had not altered in all its length. M. de Koninck 

 has also observed that he has never been able to procure a single specimen in which the 

 tube was complete, and therefore concludes that its extremity was open, and served for the 

 passage of muscular fibres, by which the animal attached itself to submarine objects. 

 But, as I have already had occasion to observe at p. 119 of my " General Introduction," I 

 feel compelled to differ with my learned friend in this last assumption. M. d'Orbigny 

 believed the prolongation to be due to malformation produced by accidental circumstances, 

 connected with the supposed constrained position in which the animal lived, which forced 

 the mantle to prolong its edges so as to reach the surface of the sea-bed ; but in the 

 localities in which the form is found a vast number of other species of the genus occur, 

 which do not present this peculiarity, so that we must regard the structure as normal, 

 although M. d'Orbigny's explanation of its function is probably correct ; and in any case 

 I cannot concur in the supposition that the shell may have been fixed to submarine bodies 

 by the means of muscular fibres issuing from the open extremity of the tube. The pro- 

 longation of the ventral valve beyond that of the dorsal one is not a feature peculiar 

 to the shell under consideration, for it has been noticed already in tw r o or three more 

 species of the same genus, as well as in a Liassic form of Thecidium, where the ventral 

 valve is prolonged much beyond that of the dorsal one. 



The two parts of which the ventral valve is composed are clearly defined by the means 

 of a groove. 



Productus ermineus, Be Koninck. PI. XXXIII, fig. 5. 



Pkoductus ekmineus, He Koninck. Descript. des Anim. foss. du Terr. Carb. de 



Belgique, p. 181, pi. x, fig. 5, 1843; and Mon. du genre 

 Productus, pi. vi, fig. 5, and pi. xviii, fig. ]. 



Spec. Char. ■ Shell of moderate dimensions, longer than wide ; hinge-line much 



longitudinal ribs of P. proboscirfeus are also much finer, numbering about fifty in the space of ten millc- 

 metres at the interior border of the visceral part. P. clavus shows no trace of sinus." It must, however, 

 be i-emembered that the American specimen is stated to have been in so imperfect a state of preservation 

 that for some time the authors hesitated in including it in their memoir, and it is still possible that 

 P. clainrs mav, after all, belong to M. de Verneuil's curious species (?). 



