380 Dr. W. B. Carpenter on the Minute Structure of 



It is not difficult for a person who has once become famiUar with 

 the appearance of this structure, to recognise it by the naked eye, 

 when it presents itself in any considerable amount. It is known 

 by its brownish -yellow colour, its elasticity (which is greater, and 

 its brittleness less, than that of other kinds of shell- structure), and 

 its prismatic or fibrous fracture. I have determined its existence, 

 by thin sections examined with the microscope, in the following 

 recent genera : Pinna, Avicula {Meleagrina) , Perna, Malleus and 

 Crenatula, belonging to the family Margaritacem ) Ostrea and 

 Etheria, which seem to be nearly related, if not to be included in 

 the same family Ostracece ; Unio and Anodon, belonging to the 

 NayadecE ; and Pandora. This last genus is the only exception 

 I have met with to the general rule, — that the prismatic cellular 

 structure is restricted to the group, of which the Margaritaceae 

 may be taken as the type or centre, the lobes of the mantle 

 being almost or completely open and the foot small ; and Pan- 

 dora is altogether so anomalous a genus, that the systematist is 

 greatly perplexed by it on other grounds than this. 



The position of Pinna among the Margaritacece (of which it 

 may be considered an aberrant form leading to the Mytilaceae), 

 the alliance of Etheria to this group rather than to the Cha- 

 macecE, and the separation of the true Ostrace<2 from the Pecti- 

 nidce on the one hand and from the Placunidce on the other, which 

 had been proposed by systematists on the ground of general cha- 

 racters, appear to me fully borne out, and indeed absolutely re- 

 quired, by the characters afforded by the structure of the shell. 



I have examined several fossil shells which exhibit similar cha- 

 racters j such as fossil Pinnce, Inoceramus, Pachymya and Ger- 

 villia, belonging to the Margaritacece ; Gryphcsa to the OstracecB ; 

 as well as Unios, &c. The prismatic cellular structure is extremely 

 well preserved in nearly all fossil shells ; and is very easily recog- 

 nised, either by fracturing the shell or by making a thin section 

 of it. The latter is necessary where it forms but a thin layer, as 

 in Gryphaa, Unio, &c. ; the former is sufficient where the layer 

 is thick enough to give the fibrous aspect on fracture. 



The determination which the use of the microscope thus enables 

 us to make, of the family to which shells must be referred whose 

 position would be otherwise doubtful, must be, I imagine, of very 

 great use to the geologist, even were that use limited to the few 

 families I have yet named, which is by no means the case. The 

 following will serve as a good example. A certain shell from the 

 lias has been described and figured by Prof. Phillips under the 

 name of Avicula cygnipes ; and the same or a closely allied spe- 

 cies by Mr. S. Stutchbury, under the name of Avicula longicoS' 

 tata. These gentlemen were not unaware of its affinity to the 

 PectinidcB, but thought that the characters of Avicula predomi- 



