390 Capt. T. Brown on some new species of the genus Pachyodon. 



nula, and other sessile Cirrliipods ; and I am much inchned to 

 believe that the real place of this perplexing group will prove to 

 be intermediate between the Conchifera and the Cirrhipoda — an 

 idea which has derived confirmation from the examination of 

 the very beautiful series of specimens in the collection of 

 S. P. Pratt, Esq., which it is to be hoped that he will take an 

 opportunity ere long of communicating to the public. 



I may state that the specimens most acceptable to me at pre- 

 sent are those of the various genera of Brachiopoda ; as I am very 

 desirous of fully investigating this group before making my first 

 Report. But I shall be very glad to receive any others, provided 

 that the name of the shell is specified, when a fragment only is 

 sent ; and the bed from which it is obtained, if it be a fossil. 



W. B. C. 



Bristol, Nov. 16, 1843. 



EXPLANATION OF PLATES XIIL XIV. 



Fig. \. Appearance of the membrane of a thin layer of the outer part of the 

 shell of Pinna, taken parallel to the surface, after removal of the 

 calcareous matter by acid. 



Fig. 2. Thin section of the outer part of the shell oi Pinna, not acted on by 

 acid ; two small black cells are seen, in which the calcareous matter 

 is deficient. 



Fig. 3. Section of Nacre, showing the wavy, but usually parallel lines, pro- 

 duced by the plaiting of the basement membrane. 



Fig. 4. Section of Avicula (?) longicostata, showing its coarsely corrugated 

 structure penetrated by tubes. 



Fig. 5. Section of the inner part of the shell of Lima rudis, showing a finely 

 corrugated structure abundantly traversed by tubes. 



Fig. 6. Section of Terehratula (recent), showing its peculiar structure, and 

 the large perforations by which the shell is traversed at right angles 

 to the surface. 



Fig. 7. Shred of Terehratula (fossil) detached by the point of a knife, from 

 a deeply- plicated specimen ; the difference of aspect between this 

 specimen and the last is entirely due (except in regard to the per- 

 forations) to the mode in which the section is made ; a natural 

 lamina being obtained in the one case, whilst in the other the plane 

 of section traverses the natural laminae obliquely. 



Fig. 8. Section of the tooth of Mya arenaria, showing a remarkable crystal- 

 line arrangement. 



XLVII. — Description of some New Species of the Genus Pachy- 

 odon. By Capt. Thomas Brown, M.W.S., M.R.P.S., Curator 

 of the Manchester Natural History Society's Museum, &c. 



[With three Plates.] 



1. Pachyodon Gerardi. PI. XV. figs. 1, 2. 



Transversely ovate, inflated, thickness equal to half its breadth ; 

 umbones produced, rounded, and contiguous; posterior side 

 short and obliquely truncated ; anterior side long and subtrun- 



