330 



ROBERT TRACY JACiSON ON THE 



is also represented by the prodissoconch in Ostrea, Avicula, Pecten, etc. (see full dis- 

 cussion, section xv). 



In studying fossil Pelccypods we find many species of Palaeozoic Aviculoids which 

 are directly comparable to the nepionic stage of Perna, PI. xxvii, fig. 1. Some species 

 of Leioptera and Leptodesma, as figured by Plall in the Palaeontology of New York, 

 Vol. V, part I, are very similar to young Perna. The most striking resemblance, how- 

 ever, is found in species of Rhombopteria (gen. nov. described in section xvi) from the 

 Silurian, as figured by Barrande. Rhombopteria glahra, sp. Mtinst. (see Barrande, PI. 

 228), is almost identical in foi'm with j'oung Perna. The form of shell found in the adult 

 of species of Khombopteria is shadowed in the stages of development of many genera 

 of the Aviculid^e and their allies as Avicula, fig. 32, p. 330; Pecten, PI. xxvni, fig. 9, 

 etc. From these close comparisons, Rhombopteria is selected as the radical from which 

 these genera are evolved as discussed in section xvi, and represented graphically in the 

 table at the end of that section. The pi'odissoconch of Perna in connnon with that of 

 Ostrea, Avicula, Pecten, etc., is the representative in the ontogeny of the individual of 



FlO. 32 



Fig. 3i- 



FiG. 32. — Youii!; Avicula sterna, showinc;,^, the prodissoconch and succeedinK nepionic stage, \iowed from the left side. 

 Fig. 33. — Same specimen viewed from the right side, showing the byssal sinus. Figs. 32-33 are drawn from tlie um- 

 bonal tip of a J'onng specimen by following the lines of growth. X 38 diam. 

 Fig. Si. — Young Avicula sterna, showing a much later growth than flgs. 32-34, X 0.3 diam. (Drawn by the author.) 



the Nuculoid radical from which the whole group evolved (section xvi), and the suc- 

 ceeding nepionic stages are referable in origin to species of Rhombopteria the next step 

 uj) in the phylogeny of the group. The prodissoconch, also the teeth, single triangular 

 cartilage pit and form of the shell in the nepionic stage of Perna show how closely 

 stages in growth may be compared to ancestral kindred and how valuable such stages 

 are in ti-acing the phylogenetic history of groups. 



The developing shell of Avicula presents characters of interest in themselves and of 

 great value in our phylogenetic studies. Thi'ough the kindness of Prof. A. E. Verrill,^ 

 I recently received some specimens of young Avicula sterna, Gould, fi-om Panama. The 

 specimens were growing thickly on the branches of Gorgonias, to which they were at- 

 tached by a byssal fixation. A well defined prodissoconch exists at the umbos of the 

 valves, figs. 32-33, and the right valve of the prodissoconch is somewhat larger than 

 the left valve. The umbos of the prodissoconch are directed jiosteriorly; it is composed 

 of homogeneous lime and shows fine concentric lines of growth. The prodissoconch of 



' I am indebted to Professor Verrill of Yale UniFersity, 

 for other specimens besides the Avicid.as mentioned, and 

 I am also indebted to Dr. C. E. Bcecher of the same in- 



stitution for many valuable specimens of developing Pel- 

 ccypods ; some of which are d.-scrib^d in this paper. 



