326 



ROBERT TRACY JACKSON ON THE 



deseribecl and figured in this pa])er. Whetlier this comparison can he properly made 

 can only be ascertained bj u studj^ of the specimen. Prajostrea is from tlie formation, 

 E. 2 and 3, Systeme Sikirien, a period equivalent to a portion of the Upper Silurian. 

 If a true Ostrea, itis the earliest appearance of the group yet recorded. Professor L. de 

 Kouinck figures a form from the Carboniferous as Ostrea nohiUisima. His figures re- 

 sendile a Perna, and are very much the form we might expect if that genus, or a similar one 

 (as Perna is uot kuowu befoi'c the Trias), should have become attached. Ostrta 'pater- 

 ctda, Winchell, from the Amei-ican Carboniferous, appears to be a true Ostrea and has 

 the typical form of the gemis. 



As it is shown that the ostrean form is due to the conditions of fixation, these early 

 and questionable species have to be looked at sceptically, as any Pelecypod adopting a 

 fixed habitat might assume a form which in isolated s[)ecimens could well be taken for 

 an oyster. The paucity of the Permian and Triassic foi'mations gives but little evi- 

 dence concei-ning the early history of the Ostreadaj, but in the Jurassic the family is well 

 developed, and from that time on there are abundant species and individuals for study. 



In the early Jurassic, Exogyra and Gryphaea are developed as well as typical Ostreas, 

 and the question comes up as to which is to be considered the typical, least modified 

 form of the fiimily. I show (below) that Ostrea, the type of the family, is connected 

 with Perna, or Perna-like forms, by important characters of anatomy and shell structui'e. 

 It has been shown that the attached valve is that which is most modified in all Pelecypoda 

 which solder one valve to a foreign body. Therefore, the same line of reasoning may 

 be followed, and as Ostrea is near to Perna, Exogyra and Gryphaea should be consid- 

 ered as extremes of variation in the OstreadjB. They are extremes of the Ostreadae as 

 Caprinula, PL xxvi, fig. 8, and Monoplenra, PI. id., fig. 9, are extremes of the Chamidse, 

 and in the same line of variation. The almost simultaneous appearance of the three 

 genera of the Ostreadae in mai'ked abundance in the early Jurassic is probably due to 

 the recognized law of quick development of new types of animals, together with the fact 

 that the Trias and Permian are formations bearing few fossils, so that what Ostreadje 

 did exist in those periods have been but fragmentaiily preserved. 



After our studies in previous chapters and the present discussion of shell form we are 

 in a position to consider the probable ancestry of the Ostreadae. The anatomy of Os- 

 trea beai's a close similarity to that of Perna. The gills of the two genera bear a close 

 resemblance.^ The palps, heart, adductor muscle, position and termination of the in- 



'Dissecting Perna iphippium I fiiul Uiat the dorsal 

 border of each pair of gills presents the cross connecting 

 lattice-like meshes chiracteristic of Ostrea. In some 

 specimens the two pairs of gills are separate from one an- 

 other thronghout their extent, as represented in PI. xxiv, 

 fig. 10, Tvliereas in other specimens the two median gills 

 are connected by concrescence at their dorsal border thns 

 uniting the two pairs of gills, as in Ostrea. The degree of 

 concrescence varied in ditterent specimens which possessed 

 it ; the gills being united for their whole extent or only 

 posteriorly. That such a difl'erence as this should exist in 

 what is commonly considered a fundamental character Is 

 peculiar and slionld be studied on fresh specimens (mine 

 being alcoholic). In a young Meleagrina, I find the two 

 pairs of gills separate, though Woodward says they are 



united behind the foot. Probably he refers to the adult. 

 In an Avicnla also, I find the two pairs of gills separated 

 thronghout their extent. In both .ivicula and Meleagrina 

 the characteristic lattice-like meshes connect tlie la- 

 mella; of either pair of gills on their dorsal border as in 

 Perna and Ostrea. The anatomy of Meleagrina and Avi- 

 cula as far as ascertained from my rather limited material 

 is as near to that of Ostrea as is the anatomy of Perna, 

 and it may be that Ostrea is a direct branch from Avicula 

 rather than from Perna I have adopted the latter view 

 because the shell of Perna approaches nearer to Ostrea 

 than does the shell of Avicnla, and because in Pernostrea, 

 an uu(|uestlona1)le Ijranch from Perna, we find a form al- 

 most identical with Ostrea. 



