PHYLOGENY OF THE PELECYPODA. 331 



Avicula is closely compai-able to the same stage of Ostrea and Perna as described, and 

 the anatomical features of this stage of Avicula are doubtless similar to those of the 

 gcnei-a mentioned. 



The early dissoconch gi'owth of Avicula is sharply defined from the prodissoconch by 

 its changed fonn and histological structure. Whereas the prodissoconch is composed of 

 homogeneous lime, the dissoconch externally is composed of prismatic cellular tissue, 

 with an inner layer of nacreous tissue (compare with Ostrea, fig. 27, p. 314). The deli- 

 cate spinose productions of the epidermis are made up of prismatic tissue like the rest of 

 the superficial laj'er of the shell. In form the nepionic stage of the dissoconch is sub- 

 rhomboidal, fig. 32; the hinge line is produced beyond the borders of the prodissoconch 

 in a straight line and anteriorly is terminated by a slightly auriculated wing; posteriorly 

 the wing is slightly roiuided at the hinge line, and is not produced beyond the free bor- 

 ders of the valves as in later stages. In the right valve, fig. 33, the byssal sinus origi- 

 nates with the introduction of the nepionic stage and is progressively pi'oduced to the 

 free border of the valves. At this stage, the developing shell of Avicula is closely like 

 the developing shell of Pecten at the same stage, PI. xxviir, figs. 9-10 and Perna, PL 

 xxvii, fig. 1. It also approaches near to the form of the young of many living and fos- 

 sil allied genei'a, and in its left valve, fig. 32, bears a close resemblance to the adult of 

 Ehombopteria, the ])i-iniitive radical of the Aviculidre (see section xvi). During the 

 later gi-owth of Avicula sterna, fig. 34, the posterior wing is gradually produced in an 

 increasingly sharper ]>osterior extension as indicated by the lines of growth in the fig- 

 iwe, and soon the full characters of the adult are introduced. The stage rejiresented 

 in fig. 34, is compared in section xvi, with the ancient genus Leptodesma, which this 

 stage probably represents in the ontogeny of Avicula. In the youngest specimen of 

 Avicula observed, the hinge teeth are similar to those of the adult, and the caitilage pit 

 is triangular, oblique and produced downwards from the hinge line of the prodissoconch 

 as in Perna, fig. 30, ]^. 329. Similar characters to those found in Aviada sterna are 

 shown in a specimen of Avicula atlantica, sp.? sent me by Dr. C. E. Beecher. 



The prodissoconch of Avicula as in Perna and Ostrea, I'epresents in development the 

 Xuculoid radical of the whole group (discussion, section xv). The nepionic stage which 

 is the next stage in the ontogeny of the individual corresponds closely with, and is the 

 representative of Rhombopteria, which is the second step up\vai"d in the phylogeny of 

 the group (see table, section xvi). The third stage in Avicula in which we find the 

 posterior wing produced and the anterior wing auriculate, fig. 34, bears a close resem- 

 blance to typical species of the genus Leptodesma which is considered as the third step 

 in the phylogeny of the group (serial steps and discussion, see section xvi). Thus 

 Avicula in its development represents very fully the phylogeny of the genus, as also 

 seen in Perna and very clearly in Pecten, section x. 



A specimen of young Mdeagrina glahra, Gould, from Xew Zealand, ^o. 584, Yale 

 University Museum Collection, has a jirodissoconch and succeeding nepionic dissoconch 

 similar to that of Avicula sterna. (Dne difterence, howevei', exists, in that the young 

 Meleagrina like the adult has no cardinal or lateral teeth as in Avicula. 



Specimens of Monotis tenuicostata, fi'om the Jurassic of the Black Hills of Dakota, 

 were loaned me by Dr. Beecher for study. The prodissoconch is preserved in both 

 valves. It has the umbos directed posteriorly, and resembles the prodissoconch of Avic- 



