Mr. S. V. Wood's Catalogue of Shells from the Crag. 455 



each other, and hence much of the secondary layer is not re- 

 solvable into distinct fibres. At certain places, however, slight 

 variations in the curves take place; they become opposed, and 

 a small depression of the outer membrane results, or the few 

 opposed curves may be large, and hence a greater hollow will 

 ensue ; but in these vessels very little matter is added to the 

 internal layer, perhaps none, and hence no dot or canal is ap- 

 parent in them (PI. XII. fig. d.). 



The spiral and sinuous direction of the fibres of the se- 

 condary layer is very easily recognizable, at least when punc- 

 tated vessels are carefully examined ; but the attention of the 

 observer should be strongly directed to it, as the brighter ap- 

 pearance of the punctations themselves, from the light only 

 passing through a less thickened layer of membrane, draws 

 more powerful consideration to them. It may also be well 

 observed when a vessel is cut, or when it breaks itself at the 

 side (as at fig./.). 



The section of a completely formed punctation is seen at 

 PL XII. fig. g; the dotted line indicates the primary mem- 

 brane, the broader dark line beneath it the fibre. 



It will be seen that we thus differ from Mohl in believing 

 that a secondarily formed fibrous layer, consisting of fila- 

 ments bent upon themselves, is mainly instrumental in the 

 formation of punctations, and agree with him in the circum- 

 stance of the depression being caused by a sinking-in of the 

 primary membrane, but which latter condition never could 

 exist without the previous existence of the fibres alluded to. 

 We also consider the fibres or filaments not to be of primary 

 development, and not as serving in this particular as a sup- 

 port to Dr. Barry, in what we have thought to have been his 

 views. (Figs. «. and b. are from Meyen.) 



XLIX. — A Catalogue of Shells from the Crag. By S. V. 

 Wood, Esq., F.G.S. 



[With a Plate.] 

 [Continued from vol. vi. p. 253.] 

 To the Editors of the Annals of Natural History. 

 Gentlemen, 



It has been my object with the present as well as the prece- 

 ding portion of this Catalogue to adhere as closely as possible 

 to the classification of Lamarck ; my only deviations from the 

 system of that eminent naturalist are those rendered necessary 

 by the additions that have been made to the science of Mala- 

 cology, and the establishment of new genera by other writers. 

 I have also myself thought it requisite to propose a few for 



