PHYLOGENY OF THE PELECYPODA. 283 



Avliich retains moi-e of the ancestral features, and to it and the free young, before mod- 

 ifying cementation takes pUice, we must turn in studying the rehxtionships of attached 

 Pelecypods. It is an animal with one side modified, losing ancestral features; the other 

 side, less modified, therefore retaining ancestral features. The actual change from an 

 unmodified fi-ee form to a modified cemented form may ])e studied in Ostrea (Pis. xxiv, 

 xxv), Hinnites and ^Inlleria (PI. xxvr, figs. 3-7) and SpondyUis (PI. xxvii, fig. 4). 

 The irregularity of attached sliells being due to the condition of fixation we may study 

 their relationships with an entirely new light on the subject, as the irregularity is a char- 

 acter superimposed on any regular form of Pelecypod by special given conditions. The 

 aberrant forms of the Chamid;e and their greatly specialized allies the Rudista; (PI. xxvr, 

 figs. 8-12 and 20) are considered as extremes of modification in the ostrean line of va- 

 riation. What the mechanical causes of this line of variations probably are, and what 

 they lead to as their highest ex[)ression is considei"ed in section vii. 



Fiudingthat irregularity commences with the introduction of the fixed period we have 

 a jiowerfnl ai-gnment in explaining the absence of features in the young dissoconch 

 stages of Ostrea which might, if free, have retained the form of related ancestral groups, 

 as is the case in the young of Perna, Pecten, Spondylus, etc. Ostrea is cemented at the 

 close of prodissoconch period so that stages of shell growth latei- than that are all mod- 

 ified. The free valve should retain more ancestral features than the attached valve and 

 this is the case with the oyster. The prodissoconch, the free valve and anatomy of the 

 adult, all unite in characters which refer the Ostreadss to a type, which is Perna, or some 

 close ally of that gemis (sections v-vii). 



I have described the development of the gills of the oyster with some care, as it is but 

 slightly treated of by other investigators of this mollusc. In order to understand the 

 serial development of the several lamellse and to show the relations of the direct and re- 

 flected borders of the filaments, diagrams are given (PI. xxiv, figs. G-12) of the de- 

 velopment of the gills of Ostrea, together with diagrams of adult gills in some related 

 genera. 



In Perna (PI. xxvii, figs. 1-3) I find a prodissoconch very closely like that of Ostrea; 

 but the initial stages of dissoconch growth show indications of a byssal notch not seen 

 at all in the prodissoconch. This character of a byssal notch originating in the initial 

 stages of dissoconch gi'owth, but not seen in the prodissoconch, is described also in Avic- 

 ula, Pecten and Spondylus (Pis. xxvii-xxviir, sections virr-xi). 



In Pecten, interesting facts are presented in regard to the life habits of crawling, bys- 

 sal attachment and mode of swimming, new points are discussed and the shell is described 

 at length in the young of five species. In all these young I found a prodissoconch with 

 important characters. The first stages of dissoconch growth in four species were found 

 to possess well defined prismatic structure in the right valve whereas this is not known 

 in the adults of these species, and in the extensive researches of Professor Carpenter on 

 shell structure, prismatic tissue was found in only one species of Pecten. The Pectens 

 are allied to a group of Pelecypoda which has prismatic structure, in the shell, and it is 

 a highly interesting fact in accord with other conclusions, that in the extreme young 

 this featiu'e exists, though early lost and not seen at all in adults. In tlie young disso- 

 conch stages of Pecten (PI. xxviii) in many species I find characters in the form of the 



