PHYLOGENY OF THE PELECYPODA. 353 



the jjrodissoconch stage, as is conclusively shown to be the case with oysters. Adult 

 Plicatala marginata, is deeply plicated ; but the plications, as in plicated oystei*s, do not 

 originate until the shell growth extends beyond the limits of the object of fixation. 



Plicatula, taken in connection with the preceding genera, is a valuable proof of the 

 correctness of the conchision that cementation destroys indications in the shells of Pel- 

 ecypoda of their ancestral relationship. Pecten, Hinnites, Spondylus and Plicatula may 

 be serially connected in the order indicated by a study of the shell, and from what I can 

 gather, the anatomical features would present a similar series. Using some license so 

 as to express it graphically, one may say: solder a Pecten when partly grown and the 

 succeeding growth is irregulai', forming the genus Hinnites, PL xxv'i, figs. 3-4; solder a 

 Pecten at a much younger period and the succeeding growth is irregular, forming the 

 genus Spondylus, PI. xxvn, fig. 4; solder a Pecten at the close of the prodissoconch 

 stage, before any dissoconch growth has taken place, and the succeeding growth is ir- 

 regular, forming the genus Plicatula, PI. xxvir, fig. 5. In all, the irregularity of growth 

 commences with the introduction of the cemented period. Spondylus is probably a branch 

 from Pecten through the intermediation of Hinnites, which unquestionably is closely 

 akin to Pecten. So near are Spondylus and Hinnites, that Stoliczka says some fossil 

 forms, first considered as Spondyli, have since been transferred to Hinnites. 



Brief studies of Lima have been made. Lima has a perpendicular mantle wall like 

 that of Pecten which is shown to be so actively functional in swimming, and it jirobably 

 subserves the same purpose in Lima, as the swimming of that genus, described by Dr. 

 Jeffreys (see Tryon), is performed similarly to that of Pecten. This difference exists, 

 however; Lima swims with the plane of the edges of valves perpendiculai', whereas in 

 Pecten they are horizontal while swimming. Prom the shape of the mantle walls and 

 shell, also from the description of the swimming movement, it appears that in Lima the 

 excurrent flow of water which propels the animal all takes place through one eared area, 

 the ventral, instead of alternately from either ear as in Pecten. In the species of Lima 

 examined, there were no guard tentacles on the free borders of the mantle walls as in Pec- 

 ten, PL xxviii, fig. 4, but Jeffi-eys' descrijition of Lima includes such organs. The margi- 

 nal tentacles at the base of the perpendicular wall are thick and numerous. Dr. Jeffreys 

 describes them as twining actively and presenting a tenacious prehensibility, grasping the 

 fingers when approached toward them. This prehensibility I have described in Pecten 

 though it is there less marked in degree. The gills of Lima have the reffected portion of 

 the filaments equal in length to the direct portion, and the tip of the reffected portion 

 of each lamina is again reflected, presenting an abbi-eviated additional lamina.^ The gills 

 of Lima differ from those of Pecten in that the separate filaments are joined by bars 

 which form a direct concrescence of the several filaments instead of by the interlocking 

 of ciliated processes, as in Pecten ; and in this character Lima agrees with Ostrea. 



A young Lima elliptica, in the Jeffi'cys' collection at the Smithsonian Institution, shows 

 a well-marked prodissoconch. In a di'awing which I made at that museum, PL xxvii, 

 fig. G, the prodissoconch is sharply marked off" from the succeeding dissoconch, and it 



' In Pecten irradians, the tip of the rctluctcd filamciits Lacazc-Piitliiers also observed siieh a brief redectioii ill 



is bent back on itself slishtly, PI. xxrv. ti-j;. 12, indicating the outer Ulanients of Auomia. 



the initial stages of such an abljreviated additional lamina. 



