POLLICIPES POLYMERUS. 309 



Later a, — The upper pair (corresponding to the interval 

 between the scuta and terga) is only a trifle larger than the 

 latera immediately beneath ; and these only a little larger 

 than those lower down. In the lowest whorl, the valves 

 are very minute, though still about twice as large as the 

 scales on the peduncle, and of a different shape from them. 

 The upper latera (viewed internally) are almost diamond- 

 shaped, owing to the prominence of the basal margin, bnt 

 this varies considerably in decree. The latera in the 

 next whorl are triangular, with the basal margins pro- 

 tuberant and arched, in a less and less degree in the lower 

 whorls, until in the lowest, the valves are elongated trans- 

 versely. 



Microscopical Structure. — A valve placed in acid leaves 

 a thick opaque mass, formed of three different kinds of 

 tissue, one having a finely shaded appearance ; a second 

 with a largely hexagonal reticulated structure, and the 

 third thin, transparent, and marked with arborescent 

 lines, which I imagine to be tubes, as will be hereafter 

 seen in Lithotrya. Near the exterior surface, there are 

 many tubuli. It appears to me probable that the strong 

 tendency which the valves in this species have to dis- 

 integrate, is connected with the unusual quantity of ani- 

 malized tissue contained by them. Externally the valves 

 are covered by a strong membrane, either white or yellow, 

 or white streaked with yellow, and marked by lines of 

 growth, and by longitudinal, sinuous, little ridges. 



Peduncle, in the upper part, of rather less diameter 

 than the capitulum; twice or thrice as long as it; tapering 

 a little downwards ; surface of attachment wide and flat. 

 Calcareous scales, minute, symmetrically and closely 

 packed together : each scale is much flattened, and its 

 shape, including the imbedded portion, is that of a spear 

 with its point broken off. The basal end of each scale 

 is conically hollow, and from the layers of growth con- 

 forming to this hollow, there is a false appearance of an 

 open tube running through the scale. 



attachment. — The surface of attachment is wide : the 



