PIIYLOGENY OF THE PELECYPODA. 345 



the lines of gi-oAvtli) P. testce and Chlamys islandica, PI. xxviii, fig. 14, agree with the 

 same stage of P. irradians, PI. xxviii, figs. 9-10, in the form of valves. This form 

 may therefore be considered as typical of the first stage of growth of the dissoconch 

 shell of the group. In studying genera of the palfcozoic Aviculidaj, we find genera that 

 agree closely in appearance with the yonng shell of Pecten; especially when it is viewed 

 from the left side as in PI. xxviii, fig. 9. The genera Actinoptera, Leiopteria and Lepto- 

 dosma, as figured in Hall's Paheontology of !N^ew York, are very comparable to this early 

 stage of Pecten. A still nearer comparison may be made in species of the Silurian ge- 

 nus Rhombopteria (gen. nov.) fig. 51, section xvi, which are closely comparable to young 

 Pecten in all the species observed. Rhombopteria is selected as the primitive radical of 

 the AviculidiB and their allies (see discussion and table, section xvi), and it is one of the 

 best proofs of the correctness of that view that the nepionic stage of Pecten so closely 

 corresponds with that early type. In the studies of Avicula and Perna, the young of 

 those genera, fig. 32, p. 330, and PI. xxvii, fig. 1, is compared closely to the same an- 

 cient genera to which I have just compai-ed young Pecten. A comparison of young Pec- 

 ten, PI. XXVIII, fig. 9, Avith the similar period of growth of young Avicula and Perna, 

 in the figures cited, shows that at this early age the thi'ee genera are closely similar. 

 Differences in the angles of contour exist, especially in the hinge line; but they are 

 slight in degree, and the imi)ortant phylogenetic conclusion may be made that these 

 genera in the nepionic stage are so alike that they are refei'able in their inherited form 

 to the same ancestral types. Thus we see that the form as well as the prismatic struct- 

 ui-e of the shell of j'oung developing Pectcns may be traced to ancestral types from 

 which the genus was evolved. 



A stage of shell growth in Pecten irradians, PI. xxviii, figs. 12-13, later than that just 

 considered, shows the beginning of features characteristic of the family and species to 

 which the young scallop belongs. Plications similar to those of the adult begin to be 

 marked in either valve and ears to be developed, as shown in fig. 12, viewed from the left 

 side, and fig. 13, viewed from the right side. The bj^ssal" notch is strongly marked, as 

 in the earlier stage, and it is to be obsei-vod that the notch is much deeper relatively in 

 this early, active, creeping stage than in the adult, where the use of the foot as a creeping" 

 organ is lost. The base of the byssal notch, fig. 13, shows fom* mai-ked, tooth-like pro- 

 cesses, of which one only was present in PI. xx\t^ii, fig. 10. These teeth may prove of 

 value in tracing relationships; but their function, if any exist, is not understood. The 

 plicati(ms of the shell are clearly defined and originate suddenly, indicating a stage by 

 their introduction. The plications are like those of the adult, fig. 37, p. 3.33, and they 

 are the same which are retained throughout life, no new plications, originating by bifui'- 

 cation or interposition, in this species. The left upper valve does not extend beyond the 

 right valve on its free margin as in the earlier stage, fig. 10. 



A later period of growth in Pecten irradians is shown in PI. xxviii,"fig. 2. The ears 

 are moi'e accentuated than in the previous figures, though they do not bear to the whole 

 shell the same relative size and shape, as in the adult, fig. 37, p. 333. The hinge line of 

 this stage, fig. 2, is much shorter than in the earlier stage, PI. xxvin, figs. 12-13, but 

 much longer relatively to the bi'cadth of the valves than in the adult. In other featui'es, 

 excepting the depth of the byssal notch, this age does not differ essentially from the adult 



