364 ROBERT TRACY JACKSON ON THE 



necessary feature of developing bivalves. Instead of being rounded in appearance as 

 in fig. 9, the prodissoconch valves appear flattened from perspective. The prodissoconch 

 of fig. 10 differs from that of the earlier stage in being chalk- white, the yellowish-brown 

 organic appearance of the earlier condition having disappeared from the wearing away 

 of the epidermis. The same is true of older specimens as long as the prodissoconch re- 

 mains intact, and it may often be found in specimens up to 4-5 mm. in length. It then ap- 

 pears as a little white cap at the umbo of either valve. The nepionic stage of fig. 10 is 

 similar to that of fig. 9, except that the lines of growth anteriorly are more crowded inward 

 toward the umljo, in this feature malsing a nearer approach to the condition characteristic 

 of the adult. Verrill notes collecting aljundant young specimens of Mytilus edulls on the 

 12th of April, and I have found them in July, Sej^tember and December, so that this spe- 

 cies evidently has wide limits in its breeding season. 



Modiola hamatus, Verrill. This species occurs on oiu" coast from Vineyard Sound 

 to Mexico. A number of young and adult specimens were collected at Martha's Vine- 

 yard and Long Island Sound, also specimens were obtained fi'om a mass of Perna 

 epMppium collected at Cuba. The young Modiola, has a well defined prodissoconch, 2>, 

 PI. XXIX, fig. 11, which is yellowish-brovvn and with umbos directed anteriorly as in 

 Mytilus. The prodissoconch of Modiola appears flatter, less rounded than the prodis- 

 soconch of Mytilus, PI. XXIX, fig. 9; but this is due in part at least to perspective as 

 noted in the older specimen of Mytilus, PI. xxis, fig. 10. The hinge line of Modiola is 

 produced posteriorly in a curving line and anteriorly the lines of growth approach the 

 nmbos in a sweeping curve, fig. 11. The form of this stage is not widely separated from 

 that found iai some species of the genus, but it more nearly ajDproaches the form of the 

 Modiomorphas, pahieozoic allies of the Modiolas. An older specimen of 3fodiola hama- 

 tus, PL XXX, fig. 1, shows the introduction of the plications Avhlch are characteristic of 

 the species. The plications originate suddenly, marking a distinct stage by (heir intro- 

 duction. 



Crenella decussata, Montagu. Youug speciniens of this species, loaned me by the 

 Smithsonian Institution, show distinct prodissocouchs, p, as shown in PI. xxx, fig. 3, an 

 umbonal view of a specimen. The prodissoconchs have rather highly developed umbos 

 which are directed anterioily and show fine concentric lines of growth. The succeeding 

 dissoconch in its initial stages of growth is already marked by radiating plications as 

 well as concentric lines of growth which are coarser than those of the 2:)rodissoconch. 



The Mytilidse are classed with the Aviculidse, as heteromyaiians, the connection being 

 based on the condition of unequal adductor muscles. This basis of classification, as pre- 

 viously considered (Introduction), is not deemed one of high, systematic importance and 

 is insufficient to connect otherwise diftering groups. The striking diff'erences in the pro- 

 dissoconch and nepionic stages of the Mytilidjc and AviculidiE are sufficient I think to 

 separate these groups, and the Mytilidae should be put in a group distinct from the Avic- . 

 nlidaj and their allies, which I have shown are all bound together by important features 

 as one group. 



Argina (Arca)pexata, Vei'rill. An extremely small specimen, PI. xxx, fig. 2, was found 

 in a drain-pipe ti-ap at Buzzai'ds Bay. The specimen was dead, but the shell was perfectly 

 preserved. The i)rodissoconch is ovally rounded, very sharply defined; and has well devel- 

 oped umbos which are nearly central. The prodissoconch shows delicate concentric lines 



