330 



ROBERT TRACY JAC.CSON ON THE 



is also represented by the prodissoconeh in Ostrea, Avicnla, Peeten, etc. (see fnll dis- 

 cussion, section xv). 



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

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

 of Leioptera and Leptodesma, as figured by Hall in the Palaeontology of JSTew York, 

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

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

 Silurian, as figured by Bnrrande. Rlwmhopteria glabra, sp. Miinst. (see Bari-ande, PI. 

 228), is almost identical in form with young Perna. The form of shell found in the adult 

 of species of Khomboptei-ia is shadowed in the stages of development of many genera 

 of the Aviculidae and their allies as Avicula, fig. 32, p. 330; Peeten, PI. xxvui, 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 prodissoconeh of Perna in common with that of 

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



Fig. 3-2. 



riG. 33. 



Fio. 31- 



Fig. 32. — Young Avirnla sterna, showing, p, the prodissoconeh and suececding nepionic stage, viewed from tlic left side. 

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

 bonal tip of a young specimen by following the lines of growth. X 3S diara. 

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



the I^iiculoid radical from which the whole group evolved (section xvi), and the suc- 

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

 up in the phylogeny of the grotip. The prodissoconeh, 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 valiiable such stages 

 are in tracing the phylogenetic history of groups. 



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

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

 I recently received some specimens of young ^4iu"c«?rt sterna, Goidd, from Panama. The 

 specimens were growing thickl}' on the branches of Gorgonias, to which they were at- 

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

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

 the left valve. The umbos of the prodissoconeh are directed posteriorly; it is composed 

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



'I am indebted to Prof essor Vcrrill of Yale University, 

 for other specimens bi'sidss the .Vvicnlas mentioned, and 

 [ am also indel)tc<l to Dr. C. E. ISeecher of the same iii- 



stitntion for many valuable specim?ns of developing Pel- 

 ecypods; soma of which are djscril)jd in this paper. 



