374 GENERAL SUMMARY TO 



adds that the punctated epidermis seems to be more easily destroyed on the true 

 Martinia than on Martiniopsis, in which it is much better and more frequeutly 

 observed. 



Family— RHYNCHONELLIDjE. 



This is a most important family among the Brachiopoda, and has been treated 

 of at great length in the pages of this Monograph. It seems also to be one of the best 

 circumscribed. Dr. Waagen, in his work on the Salt Range, proposes some alterations 

 in the classification of the genera composing it. At page 409, he says " that if the 

 interior of the different shells were better known, a greater number of well-defined 

 genera would be distinguishable in this family also as well as in the Terebratulida and 

 Spiriferida. As it is, there can be distinguished at once three large groups of forms 

 among the shells belonging to the family. The first is characterised by a very insignifi- 

 cant development of internal septa. In the ventral valve generally only dental plates are 

 present, which do not unite in the middle line, and in the dorsal valve a not very conspicuous 

 median septum is developed. In the second group of forms the internal septa are much 

 more strongly developed and have gained much in extent. The dental plates converge 

 towards the middle and form also in the ventral valve a more or less extended median 

 septum. The septum in the dorsal valve is also generally rather large and high, and 

 bears often at its upper end shelly expansions. In the third group at last the internal 

 septa have reached their maximum of development. The whole inside of the shell is 

 divided by them more or less distinctly into five chambers, one median, marked off by 

 the dental plates of the ventral valve, and the lateral expansions of the two septa in the 

 dorsal valve, and two lateral chambers outside of the median septa in both valves." These 

 divisions well correspond to Waagen's three sub-families Rhi/nchoncttida, Camarophorida, 

 and Penfamerina. 



The RhynchonettidcB comprise a very large and most perplexing number of species, 



and waved in some specimens, but more often straight in others." These lines have been often observed 

 and figured, " but, so far as I am aware, no one has noticed or described these lines or tubes as forming 

 and corresponding with a row of denticles along the edge of the valve in the specimens on which they are 

 found. . . . Some of the lines bifurcate in their upper extremity, but the greater majority are single, 

 and they slightly increase in thickness as they approach the edge of the hinge, in which they are seen to 

 be continuous with the denticles. ... I am now inclined to regard them as having been originally 

 composed of vertical lines of aragonite in the shell-structure, which, being harder than the ordinary 

 calcite of the shell, went to the formation of the row of denticles. These lines are now seen to be 

 composed of a coarser and more structureless kind of calcite than that forming the hinge-area of the shell, 

 and it often weathers into hollow grooves upon the external surface, or when the lines have been etched 

 with acid," &c. In his paper Mr. Young enters upon many details in support of his views, but which 

 space will not enable us to transcribe. 



