326 



E015KRT TRACY JACKSON ON THE 



described and figured in this paper. Whether this enmpai'ison can be properly made 

 can only be ascertained by a study of the specimen. Pi-seostrea is from the formation, 

 E. 2 and 3, Systeme Sihirien, a period eqnivalent to a portion of the Upper Sihirian. 

 If a trne Ostrea, it is the earliest appearance of the gronp j'et recorded. Professor L. de 

 Koninck figures a form from the Carboniferous as Ostrea nohillisima. His figures re- 

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

 (as Perna is not known beibi'e the Trias), should have become attached. Ostrea j^ater- 

 cida, Winchell, from the American Carboniferous, appears to be a true Ostrea and lias 

 the ty]ncal form of the genus. 



As it is sh(nvn that the osti'ean foi"m 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 specimens could well be taken for 

 an oyster. The [laucity of the Permian and Triassie formations gives but little evi- 

 dence concerning the eaily history of the Ostreadje, but in the Jurassic the family is well 

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



In the earl}^ Jurassic, Exogyra and GryphiBa 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 famil3^ I show (below) that Ostre:i, the type of the family, is connected 

 with Perna, or Perna-like forms, by important characters of anatomy and shell structure. 

 It has been shown that the attached valve is that which is most modihed in all Pelecy[)oda 

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

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

 ered as extremes of variation in the Ostreadjc. They are extremes of the Osti'eadae as 

 Capi'inula, PI. xxvi, fig. 8, and Monopleura, PI. i(Z., fig. 9, are extremes of the Chamidje, 

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

 genera of the Ostreadie in mai-ked abundance in the early Jurassic is probably due to 

 the recognized law of quick development of new types of animals, together Avith the lact 

 that the Trias and Permian are ft)rmations bearing few fossils, so that what Ostreadae 

 did exist in those periods have Ijeen but fragmentarily preserved. 



After our studies in previous chaptei's and the present discussion of shell form we are 

 in a ])osition to consider the pi'obable ancestry of the Osti'eadre. The anatomy of Os- 

 trea bears 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 fphippium I find Uiat the dorsal 

 border of each pair of gills presents the cross connecting 

 lattice-like meshes th iracteristic of Ostrea. In some 

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

 other thronghont their extent, as represented in PI. x.xiv, 

 fig. 10, whereas in otlier specimens the two median gills 

 are connected by concrescence at their dorsal border thns 

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

 concrescence varied in different specimens which possessed 

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

 posteriorly. That such a diflference as this should exist in 

 ■what is commonly considered a fundamental character is 

 pecnliar and .should 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 

 throughout their extent. In both .iricula and Meleagiina 

 th3 characteristic lattic:;-lik3 mashes connect the la- 

 mellic of either pair of gills on their dorsal border as in 

 Parna and Ostrea. Th > anatomy of M'leagrina and Avi- 

 cnla 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 

 rathar than from Perna I have adopted tha latter view 

 because the shell of Perna approaches nearer to Ostrea 

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

 an nncjuestionable branch from Perna, we find a form al- 

 most identical with Ostroa. 



