PHYLOGENY OF THE PELECYPODA. 



383 



well developed. Provisionally, Cassianella is considered a branch from Avicula, in 

 the same line as Pseiuloinonotls, etc. (Further description oF Cassianella, see p. 332.) 

 Vulsella, when 3"oung, fig. 36, p. 332, bears a considerable resemblance to young Avic- 

 ida, fig. 32, p. 330, as it has a similar prodissoconch and nepionic rhombic form. La- 

 ter, the resemblance is lost, as wings are wanting, the hinge line is highly reduced and 

 often produced in a curve around the deeply-sunken caitihige pit. Vulsella is evidently 

 a side-issue from the Avicula stock and provisionally is placed on the same branch as 

 Malleus. 



Malleus is a genus closely allied to Avicula. The main features of difference are the 

 great development of prismatic cellular tissue, loss of teeth, deep cartilage pit and con- 

 torted form. In the young it resembles Avicula, as pointed out by Woodward. It 

 should be observed, however, that it is the young of Avicula, fig. 32, p. 330, which the 

 young Malleus most nearly resembles, as shown at the umbonal region or early growth 

 of fig. 52. The irregular, contorted growth of adult Malleus, PI. xxvrr, fig. 19, and 



also the relative great development of prismatic 

 cellular tissue and extreme reduction in area and 

 thickness of nacreous tissue, are features of the 

 adult which point toward the probability that Mal- 

 leus is a degenerate or retrograde genus. 



On another line of descent we find Meleagrina. 

 "When young it resembles young Avicula, fig. 32, 

 p. 330, but it has then no teeth. The cartilage pit 

 descends from the limits of the prodissoconch as 

 in Avicula and Perna, fig. 30, p. 323. With in- 

 creasing age the oblique form of the body of the 

 shell decreases and the characteristic rhombic form 

 of the adult progressively increases. The rhom- 

 bic form characteristic of the adults of this and 

 some other genera is not necessarily to be considered as an inheritance from Rhombop- 

 teria. It is rather an approach to a form normal to the group and which is frequently 

 rej)eated in genera not closely akin. (Further description of Meleagrina, see p. 331.) 

 Perna is an unquestionable branch from Avicula. The nepionic stage of Perna, Pl.xxvn, 

 fig. 1, is similar to the same stage of Avicula, fig. 32, p. 330, the main ditfereuce being that 

 the anterior auricle descends directly from the prodissoconch without an anterior exten- 

 sion of the hinge line. At this age but a single triangular cartilage pit exists, fig. 30, 

 p. 329, and cardinal and lateral teeth like those of Avicula underlie the hinge Hue. 

 Later, new cartilage pits develop on the hinge line, fig. 31, p. 329, and in successive 

 growth they assume the perpendicular form characteristic of the pits of the genus, PI. 

 XXVI. fig. 16. The numbei" of pits is variable, but twenty or more exist in some fossil 

 species. Wings may or may not develop in Perna, PI. xxvi, figs. 16-18, but the body 

 of the shell is produced obliquely or vertically as in Avicula or Malleus, PI. xxvr, fig. 19. 

 In no species are teeth known in the adult and they seem to be purely hereditary char- 

 acters limited to early stages of development. The microscopic shell structure of Perna 

 is identical with that of Avicula, and the anatomy of the two genera is alike as far as 



Fig. 52. — Malleus viilgarU, L. Tip of left valve 

 showing the form of the young. X 13 diam. 

 (Drawn by the author.) 



