

PHYLOGENY OF THE PELECYPODA. 325 



is even and regularly pecteiiiform like the right valve. When the right valve becomes 

 attached, the left valve aftected somewhat by the same conditions loses much of the 

 regularity and evenness of marking characterizing its nealogic period, l)ut it never be- 

 comes as irregular in growth and ostreaform as does the right (attached) valve. Hin- 

 nites is marked therefore by a byssated pecteniform stage followed by an attached os- 

 treaform stage. The transitions described in Hinnites cortesll, I hav^e also observed 

 in the living species H. gigantea, Gray, and H. sinuosus, Gmelin. 



Ostrea we have shown becomes attached at the close of tht; prodissoconch period, and 

 the same is true of Plicatula (see section xi) ; as soon as attached both these genera, as 

 in Hinnites and Mnlleria, assume the ii-regnlar ostrean form of shell, losing entirely the 

 symmetry in the valves of the early unattached period. 



Pernostrea, a Jurassic genus, is of interest in these studies. According to Stoliczka 

 the shell is "rounded or oval, solid, more or less tumid, inequivalve, the left valve being 

 in adult specimens attached; structui-e lamellar, resembling that of Perna 

 Again Stoliczka says: "This genus forms a connecting link between Melina (Perna) 

 and Ostrea, differing from the former especially by its sessile habitat, absence of a bys- 

 sal sinus and strongly excavated muscular scar; from the latter by the presence of sep- 

 arate ligamental grooves. Externally Pernostrea is barely distinguishable from Ostrea." 

 From our point of view, the ostrean form is due to the condition of fixation. Therefore 

 Pernostrea is like Ostrea because it is cemented to foreign bodies. I have shown that 

 in Hinnites, PI. xxvi, figs. 3-4, the byssal notch disappears as soon as the shell becomes 

 soldered to an object of fixation, as the byssal attachment at that period is abandoned 

 and the shell assumes an irregular ostrean growth. Therefore the disappearance of a 

 byssal notch in Pernostrea is another chai-acter fairly attributable to the condition of 

 cemented fixation. In the species of Perna in PL xxvr, figs. 16-18, we find that from the 

 typical form of this genus having a long hinge area and many cartilage pits, fig. 16, a se- 

 I'ies may be traced to a form with few cartilage pits and wing-like productions of the 

 shell, fig. 18. If we should carry this reduction of the cartilage pits still further till only 

 one remained while still retaining the wing-like productions of the shell we should have a 

 form similar to the nearly allied genus Malleus. Still more to the point is the fact that 

 in studying young Perna I have found that at first only one cartilage pit exists, which is 

 triangular in outline and oblique in position as in Meleagrina, Malleus and Ostrea. It 

 seems that there is no diffisulty in a single cartilage pit being derived from many car- 

 tilage pits; so, that other points being considered, Pernostrea may very properly be con- 

 sidered as intermediate in form between Perna and Ostrea. As Pernostrea is known 

 only in the Jurassic, and true oysters are known from older formations, it is best to con- 

 sider Pei'uostrea not as in the line of ancestry of the Ostreadtc, but as a separate branch 

 from Perna parallel to the branch on which the Ostreadaj evolved as discussed at the 

 close of this section and in section xvi. 



In looking back to the earliest known oysters, we find that they are very typical, in- 

 equivalvular ostrean forms. M. Barraude's Prceostrea hohemica, Barr., a form which 

 seems referable to the oysters, is based on isolated valves attached by the umbo of appar- 

 ently the left valve. His PI. Ill, fig. 2, is an upper free valve and bears at the extreme 

 tip a rounded, very marked early stage which resembles the ostrean prodissoconch as 



MliMOIItS UOSTON SOC. NAT. HIST., VOL. IV. 41 



