400 JACKSON ON THE PELECYPODA. 



Fig. 4. Anomia glabra, very yonng, viewed from the left side : p, i)rodissoc()iich, ■\vhlcli Is on the border of the valve, 

 not yet beins enclosed by the dissoconch growth. X 50 diara.. p. 3511. 



Fig. 5. Y<niug Anomia glabra, viewed from the left side : p, the prodlssocouch. is uot yet surrounded Ijy dissoconch 

 growth. Compare with tigs. 4 and 8. The dissoconch is sub-oval and an extensible foot,/, exists. X 50 diam., pp. 355, 389. 



Fig. li. Anomia, same specimen as flg. 5, viewed from right side: p, the prodissocouch. shows indications of a byssal 

 sinus in the Hues of gi'owth. The dissoconch is extremely thin, surrounds the byssal notch and anteriorly has grown 

 around the prodissocouch covering its umbo. The method of growth is indicated by the lines of growth, pp. 35'J, 388. 

 Compare with Pecten, PI. xxvm, fig. 10. 



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

 notch which is accentuated in the succeeding dissoconch growth : tlie latter finally grows around the Ijyssus so as to on- 

 close it. Compare with tig. G, this plate, also with PI. xxvii, flg. 3, and PI. xx■^^^, flg. 10. X 50 diam., p. 3G0. 



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

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

 diam.. [ip. 3511, 389. Specimens of Anomi*, figs. 1-8, were collected at Buzzards Bay, .Vugust, 1888. 



Fig. 9. Jlytihis edulis. very young : p, the prodissoconch, succeeded by early nepionic growth. Beach Blurt'. JIass. 

 X 50 diam., p. 303. 



Fig. 10. Mytilus ednlis. oUler than flg. 9 ; p, prodissocouch, succeeded by nepionic growth. Nantucket. X 47 diam. 



Fig. 11. Young Modiola hamatus : p, prodissoconch, succeeded by a smooth nepionic stage. Later, the smooth ne- 

 pionic stage is succeeded by the plicated gi'owth typical of the species as shown in PI. xxx, flg. 1. Specimen on oyster 

 shells from Baltimore, collected at Martha's Vineyard. X 50 diam., p. 364. 



F'igs. 3, 5. (i. 9. ID and 11 drawn bv the author: all others bv Mr. .T. II. Emertou. 



PLATE XXX. 



F''ig. 1. Modiola hamatus, older than PI. xxix. flg. 11 : /). prodissoconch, succeeded by an early plain nepiouic stage, 

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

 X 50 diam., p. 3(;4. 



Fig. 2. Argina pexaCa, very young, snowing a prodissoconch. p. and succeeding dissoconch growth. Buzzards Bay, Sep- 

 tember, 1888. X 87 diam., p. 3(;4. 



Fig. 3. Crenella decussata : p, prodissoconch, succeeded by the early dissoconch growth. Specimen from Smithsonian 

 Institution. X 23 diam., p. 3(i4. 



Fi"'. 4. Sphxrium securis : embryo from gills of the parent. The whole shell of this specimen represents the prodisso- 

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

 phon. Medford, Mass., Nov. 1S8H. X about (30 diam., p. 369. 



Fl". 5- Sphmrium securis, a little older than flg. 4, viewed from left side : p, prodissoconch, succeeded I)y early dissoconch 

 growth. X about 25 diam., p. 369. 



Fig. 5a. Sphwriiini^securis, a little older than figure 5, viewed from right side: p, prodissoconch. X 8 diam., p. 370. 



Fig. 6. Young Petricolapholadiformis, showing p, prodissoconch and succeeding nepiouic stage which is strikingly difl'er- 

 eut from later stages:/, foot; m, mantle border; st, siphons, both with simple, unbranched tentacles. X 47 diam., p. 370. 



Fig. 7. Petricola pholadiformis, older than fig. 6, shows early nepionic smooth stage succeeded by the later characteristic 

 plicated and elongate^features typical of the adult. X 8 diam., p."371. Figs. 6-7, of specimens fi-om Nantucket, July, 1889. 



Fig. 8. Petricola, tip of branchial siphon of a specimen of aliout the age of fig. 7, showing the arborescently branching 

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



Fig. 9. Young Fe)i!(s wicj'cejwn'a : j>, prodissocouch, succeeded by early dissoconch growth. X 87 diam., p. 371. 



Fig. 10. Venus mercenaria, an older specimen than fig. 9. showing a prodissoconch p, and succeeding nejiionic growtli. 

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



Fig. 11. Scrobicularia teKWis : p, prodissoconch succeeded by early dissoconch growth. The prodissoconch is not so 

 sharply defined as in many genera, because the early dissoconch is not characterized by markedly differing features. Speci- 

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



Fig. 12. Mya arenaria, very young: p, prodissoconch, and well defined succeeding dissocouch. Buzzards Bay, 1888. 

 X 87 diam., p. 374. 



Fig. 13. Mya arenaria, older than fig. 12 : a prodissocouch, p, less clearly defiuetl 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 excurreut and a short nou-exteusible incurreut tube. 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. 



Fio-s. 4 to 8 inclusive, and fig. 14 drawn by the author; all others drawn by Mr. J. H. Emertou. 



