400 JACKSON ON THE PELECYPODA. 



Fii;. 4. Anomia glabra, very youiii;. vie-\vc(l fi-oin tlu' left sick': ;). prodis^ocomli, wliicli is dh tlic bonlur of the valve, 

 not yet beinir enclosed by the dissoconch jirowth. X .")0 diaiii., p. :','>'■>. 



Fig. 5. Youns Anomia ylahra. viewed from the left side: p, the prodissoeouch, is iKjt yet siuTouiided by dissoeouch 

 jinnvth. Compare with tins. + and 8. The dissoconeh is sub-oval and an e.xtensible foot,/, exists. X 50 diani.. pp. 855, 389. 



Fill. (>. Anomia, same speeimen as fiir. 5, viewed from right side; p, the prodissoconeh. shows indications of a byssal 

 sinus in the lines of groM'tli. The dissoeouch is extremely thin, surrounds the byssal notch and anteriorly has grown 

 around tlie prodissoeouch covering its umbo. The method of growth is indicated by the lines of growth. ]>[>■ 35it, 388. 

 Compare with Pccten, PI. .x.xviii, fig. 10. 



Fig. 7. Young Anomia glabra, umboual region of right valve: p, the prodissoeouch, shows indications of a byssal 

 notch which is accentuated in the succeeding dissoconch growth ; the latter finally grows around the byssus so as to en- 

 close it. Compare with tig. IJ, this plate, also with PI. xxvii, tig. 3, and PI. xxxm, lig. 10. X 50 diam., p. 300. 



Fig. 8. Anomia, tip of valve, older than tigs. 4 and 5 and in the same view. The dissoconch growth has now enclosed 

 the prodissoeouch, p, and the borders of the opposing sides form a marked area by their ingrowth at the umbo. X 50 

 diani., pj). 35!), 389. Specimens of Anomia', tigs. 1-8, were collected at Buzzards Bay, August, 1888. 



Fig. 11. Mijtllus edulis, very young: ;;, the prodissoeouch. succeeded by early uepionic growth. Beach Blutt'. JIass. 

 X ."jO tliam., p. 3G3. 



Fig. 10. Mijtilus ednlis, older than tig. 9 ; p, prodissoconeh, succeeded by uepionic growth. Xautucliet. X 47 diam. 



Fig. 11. Youug Modiola hamatus: p, prodissoconeh, succeeded by a smooth nepiouic stage. Later, tlie smooth ne- 

 pionic stage is succeeded by the plicated growth typical of the species as shown in PI. xxx, tig. 1. Specimen on oyster 

 shells from Baltimore, collected at Martha's Vineyard. X .50 diam., p. 3G4. 



Figs. 3, 5, fi, 9,' 10 and 11 drawn by the author; all others by Mr. .T. H. F^mertou. 



PLATE XXX. 



F'ig. 1. Modiola hamaltis, older than PI. xxix, flg. 11 : p, prodissoconeh, succeeded by an early ijlaiu uepionic stage, 

 which is again succeeded by a plicated later stage. Attached to a cluster of Pernas in Museum of Comparative Zoology. 

 X 50 diam., p. 304. 



Fig. 2. Argina pexata, very young, showing a prodissoeouch, p. and succeeding dissoeouch growth. Buzzards Bay, Sep- 

 tember, 1888. X 87 diam., p. 3(j4. 



Fig. 3. Crenella decussata: p, prodissoconeh, succeeded by tlie early dissoconch growth. Specimen from Smithsouiau 

 Institution. X 23 diam., p. 3t;4. 



Fi"'. 4. Sphcerium seciiris : emljryo from gills of the parent. The whole shell of this specimen represents the prodisso- 

 coneh period: a. ad, anterior adductor; p. ad, posterior adductor; g, gill tilaments ; pi. palps;/, foot; ot, otocyst: si, si- 

 phon. Medford, Mass., Nov. 1888. X about (!0 diam., p. 369. 



Fis. 5. Sphxrium seairis, a little older than tig. 4, viewed from left side : p, prodissoconeh, sui:ceeded l)y early dissoconch 

 growth. X about 25 diam., 'p. 3(;9. 



Fig. 5a. Sphccrium^seeuris. a little older than figure 5, viewed from right side : p, prodissoconeh. X 8 diam., p. 370. 



Fi". li. Youutr Petricolapholadiformis, showing p, prodissoconeh and succeeding nepiouic stage which is strikingly difter- 



ent from later stages;/, foot; «», mantle border; si, siphons, both with simple, unljranched tentacles. X 47 diam., p. 370. 



Fiy. 7. Pelricoln pholadiformis, older than tig. G, shows early uepionic smooth stage succeeded hy the later characteristic 



plicated and elongate features typical of the adult. X 8 diam., p. 371. Figs. 6-7, of specimens from Xantueket, .July, 1889. 



Fisj. 8. Petricola, tip of branchial siphon of a specimen of aljout the age of tig. 7, showing the arboresceutly liranching 



tentacles which surround the orifice. X 19 diam., p. 371. 



Fi,"'. 9. Young Vemis mercenaria ; p, prodissoeouch, succeeded by early dissoconch growtli. X 87 diam., p. 371. 

 Fi". 10. Venus mercenaria, an older specimen than fig. 9, showing a prodissoconeh p, and .succeeding uepionic growtli. 

 X 40 diam., p. 371. Figs. 9-10 from Buzzards Bay. 



Fisi. 11. Scrobicularia JenHjs ; p, prodissoeouch succeeded by early dissoconch growth. The prodissueoncli is not so 

 sliar|)ly defined as iu many genera, because the early dissoeouch is not characterized liy markedly <litteriug features. Speci- 

 men from Smithsonian Institution. X 50 diam., p. 372. 



Fig. 12. Mija arenaria, very youug: ;;, prodissoconeh, and well defined succeeding dissoeouch. Buzzanls Bay, 1888. 

 X 87 diam., p. 374. 



Fi". 13. Mya arenarin, older than fig. 12 ; a prodissoconeh, p, less clearly defined than when younger, exists at the 

 umbos. The dissoconch growth has acquired characteristics of the genus in form. A large active foot exists. The 

 siphon consists of an extensible excurrent and a short non-extensible incurrent tnbe. Buzzards Bay. X 20 diam., p. 372. 

 Fig. 14. Siphon of a young clam older than fig. 13. The massive siphon is fringed with tentacles. The incurrent 

 tube is also bordered by tentacles, but the excurrent tube bears none ; it is thin walled and retractile : s, border of shell 

 from which the siphon extrudes. Chelsea Beach. X 10 diam., p. 373. 



Fi^s. 4 to 8 inclusive, and fig. 14 drawn by the author; all others drawn by -^Ir. ,1. II. Euurton. 



