THE NAUTILUS. Ill 



03 is at first continuous with alii, and in the left valve 02 

 with all, C4 with a subsequently suppressed alV (B. B. 

 Woodward, 1913, p. 3). 



The mode of formation as set forth has not been seen in 

 any of the hundreds of young nepionic mussels carefully ex- 

 amined, of many species of Pisidium, Sph&rium and Muscu- 

 liurn. This does, of course, not mean to deny its existence in 

 other forms of Eulamellibranchia. Those under considera- 

 tion may be somewhat aberrant. 



In the very young shelly nepionic valves, deposited on the 

 primitive continuous shell membrane to be the periostracum, 

 the initial hinge-plates are formed centrally, one in each 

 valve, callus-like, and growing out towards either end (pi. 3, 

 fig. 9). At a somewhat later stage (fig. 10) the cardinals are 

 forming, as small nodules, and the laminge al and all, pi pi 

 and pll are beginning to form. Of alii and pill, there are 

 not even "vestiges"; they appear later, at least in some in- 

 stances, apparently branching out from al and pi, when C3 

 has grown somewhat, is generally larger than alii, and well 

 confined. From this it is evident that the right cardinal can- 

 not be the "internal" or proximal end of alii ; so far as 

 seen, the two are never connected. 



In the left valve, as stated, 02 is present, and well confined 

 before there is really an anterior lamina, and so it also can- 

 not be the internal, and last formed, end of all. In some 

 Pisidia, however, e. g. pauperculum, with a peculiar thicken- 

 ing of the hinge-plate in the adolescent and adult, all be- 

 comes subsequently connected with 02; but in other speci- 

 mens of the same, the proximal end of C2 curves upward 

 and stands above 02, without connecting. In Musculium, 02 

 is often prolonged anteriorly, at it base, and is parallel to the 

 proximal part of all. Of an early formed and subsequently 

 suppressed alV, not a vestige has been seen, and 04 has 

 been growing out on the plate at the same time with 02 

 and 03. 



It should also be noted that nothing of a provinculum has 

 been seen in any specimen, and that stage of development 

 appears to have been skipped in the Sphccriida. 



