Miscellaneous. 485 



Note on the Shell-structure of certain Naiades. 

 By C. A. White, M.D. 



Those who have given attention to fossil Lamellibranchiates can- 

 not fail to have observed that, whenever the shell-substance is pre- 

 served at all, it is universally thin, even in large specimens — too 

 thin, indeed, to have given sufficient protection to the moUusk which 

 it enclosed. It seems evident, therefore, that some portion of the 

 shell-substance must have disappeared during the process of 

 fossilization. Thus those fossil shells which possess a prismatic 

 outer layer usually have this layer alone preserved, at least when 

 they occur in calcareous strata. Numerous specimens of Myalina 

 subquadrata from the Upper Coal-measures, and Inoceramus proble- 

 maticus of the Cretaceous of Western Iowa, illustrate this well, in 

 which the interior markings are uniformly obliterated. Wliile 

 lately examining the Naiades of the Iowa river I observed that they 

 all possessed a prismatic outer layer of a character not distinguishable 

 from that of Myalina subquadrata. The prisms are of about the 

 same size in each ; but those of the Inoceramus are nearly twice as 



The following species have thus far been found to possess the 

 characters referred to: — Unio ulatus, Say, U. cornutus, Barnes, U. 

 crassus, Say, U. ebenus, Lea, U. pUcatus, Say, U. rectus. Lam., U. 

 securis, Lea, U. tuberculatus, U. ventricosus, Barnes, Alasmodonta 

 complanata, Barnes, A. truncata, Say, and an undetermined species of 

 Anodonta. 



This comprises all the genera (or subgenera) found in our region ; 

 but it is not improbable that the prismatic structure is common to 

 the whole family. The prismatic layer is of about the same thick- 

 ness in all, the Anodontas and Alasmodontas being thinned at the 

 expense of the inner or structureless layers ; it is from haK a 

 millimetre to more than a millimetre in thickness, the epidermis 

 resting directly upon it. Except in very old specimens, this layer 

 constitutes the whole thickness of the shell for the width of a 

 couple of millimetres from the margin ; and in very young specimens 

 it exclusively occupies about half the space between the margin and 

 the pallial line. 



The prismatic structure may be detected by the naked eye, and 

 can be well studied by the use of one of Tolles's |-inch triplet 

 lenses. Under this magnifier the inner surface of the marginal 

 border is seen to be finely granular, each granule being the con- 

 vex end of one of the prisms which constitute the layer. These 

 prisms extend through the layer to the epidermis without inter- 

 ruption, reminding one, in their appearance upon fracture, of certain 

 species of Chcetetes. But viewing the prisms upon their ends they 

 are seen to vary in size much more than the cells of Chcetetes do, 

 and consequently to lack that uniformity in cross section presented 

 by the coral. 



If our Naiades were fossilized under the same circumstances that 



