LIST OF PLATES. 



Plates VI., VII., VIII., IX. are designed to illustrate variations of form 

 between individuals of the same species, observed in comparing large 

 numbers of specimens from the Reigen Collection of Mazatlan Shells: vide 

 Report, pp. 24-1 -264-. 



PLATE VI. 



Fig. 1. Three adult specimens of Area grandis, Brod. & Sby., laid on the same hinge- 

 line : n, normal state ; e, elongated ; o, obese. 



Fig. 2. The same specimens in profile. 



Fig. 3. Two young specimens, showing that the changes of form are not merely the 

 result of circumstances of growth : e, elongated ; t, transverse. 



Fig. 4. The same specimens in profile. The A. cequilatera, Sby., is probably the 

 young of this species. It has been selected from a group usually constant 

 in its characters; the nestling Byssoarks being notoriously irregular. 



PLATE VII. 



Fig. 1 a. Cyrena Mexicana, Brod. & Sby. Two young specimens laid together at 

 the left angle between the dorsal margin and the umbo : n, normal ; e, 

 elongated. In this state it forms part of C. Floridana, Desh. MS., non 

 Conr. 



Fig. 1 b. Four specimens, similarly placed, adult : n, the largest shell, normal shape ; 

 e, elongated ; r, rounded ; a, an extreme form, described by Dr. Gould as 

 C. altilis. The Cyrence are generally very regular shells. 



Fig. 2. Two specimeus of Avicula sterna, Gould : the black line, normal; the dotted 

 line, with the characteristic tail almost evanescent, while the upper ears 

 are enormously developed. 



Fig. 3. Gadinia pentegoniostoma, Sby. : a, normal state, round, margin deeply 

 crenate, ribs deeply grooved internally ; these characters pass away more 

 or less in the other specimens ; b, with one corner ; c, with two corners ; 

 d, with three corners ; e, with four corners ; /, with five corners ; g, with 

 six corners obscurely marked. 



Fig. 4. Glyphis incequalis, Sby., including Fissurella pica, Sby., and F. mus, Rve. : 

 a, extreme form, type of F. incequalis, oblong, with faint sculpture, shown 

 at a', and trilobed hole ; b, lobes of hole evanescent ; c, form F. mus ; 

 d, type of F. pica, oval, with rounded hole and strong sculpture shown at 

 d' ; e, f, g, h, i, k, I, m, n, internal views of the hole and callosity, mag- 

 nified, showing the great changes of form, and the development or absence 

 of the posterior truncation and pit. This, with an oval hole, are con- 

 sidered generic characters by Messrs. H. & A. Adams: vide Gen. vol. i. 

 p. 447 (as Lucapina, but not of Gray, except L. cremdata). 



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