complexity of the hinge apparatus of the prodis- 

 soconch. The hinge ensures the exact closui-e of 

 the valves and prevents them from sliding on each 

 other under uneven pressure. Consequently, as 

 the larva grows the hinge apparatus increases in 

 strength and complexity. According to Bernard 

 (1898), who made an extensive study of the 

 ontogeny and morphology of larval shells of 

 bivalves, the straight part of the dorsal shell 

 margin thickens to form a provinculum (from the 

 Latin "pro", before, and "vinculum", bond or 

 band) or primitive hinge. The provinculum (by 

 definition) always bears teeth or is shaped into 

 toothlike projections which fit into the correspond- 

 ing gaps of the opposite valve. 



On the basis of the liinge structure Rees (1950) 

 proposed a system of classification of bivalve 

 larvae that greatly facilitates their recognition 

 in plankton samples (fig. 345). He postulated 

 that each superfamOy of bivalves has a distinct 

 type of larval hinge; that the shape of the hinge 

 is typical as a generic and species characteristic; 

 and that the texture of the larval shell can be used 

 in certain cases in the recognition of a species. 

 In the families Pteriacea and Ostreacea the hinge 

 apparatus consists of a series of small, uniform 

 teeth (taxodont teeth) in the central portion of 

 the strip and a few larger rectangidar teeth with 

 clear gaps between them (fig. 346) at the posterior 

 section. The distinguisliing features of the species 

 of these families is the absence of lateral and 

 special teeth and of flanges, i.e., the thick edges 

 of the valves on both sides of the provinculum. 

 The hgament lies between the posterior rectangular 

 teeth and the taxodont strip (Bernard, 1898; 

 Borisiak, 1909). In 0. edulis there are some large 



RIGHT 



Figure 34.5. — Type of hinge of the families Ostreacea and 

 Pteriacea. Redrawn from Rees, 1950. 



364 



Figure 346. — ^Drawing of a 5- to 6-day-old prodissoconch 

 of C. virginica, 70 m long, examined from the dorsal 

 side. 



corrugations anterior to the taxodont teeth (Rees, 

 1950), but their taxonomic value is doubtful. 



Differing arrangements and numbers of taxodont 

 teeth in the shells of various species of oyster 

 larvae are used for their identification. The 

 straight-hinge line of a 5- to 6-day-old larva of 

 C. mrginica grown in laboratory culture has two 

 groups of rectangular teetli that can be clearly 

 seen by examining the shell from the dorsal side 

 (fig. 346). At this stage there is only a slight 

 difi"erence between the upper (riglit) and lower 

 (left) valves. The difl^erence becomes more 

 pronounced as the larva reaches the umbo stage. 

 In a series of papers Ranson (1943, pp. 52-58) 

 attempted to establish the classification of all 

 adult Ostreacea on the basis of the fully developed 

 prodissoconchs. Essentially this work was based 

 on the investigations pubhshed long ago by Bernard 

 (1898) and Borisiak (1909). Ranson (1960) 

 separates the oysters into three genera: Pycno- 

 (lonte, Crassostrea, and Ostrea. Each genus, accord- 

 ing to his data, is determuied by the character of 

 the final prodissoconch hinge and tlie position of 

 the ligament in relation to the hinge. He con- 

 cludes his paper with the statement that "as far 

 as the Ostreidae are concerned, the species can now 

 be established on a firm basis, which so far had 

 never been done by studying the adult." The list 

 publislied by Ranson includes 5 species of Pycno- 

 donte, 12 species of Crassostrea, and 19 species of 

 Ostrea. Unfortunately the diagnosis is given only 

 for each genus without descriptions of taxonomic 

 cliaracters which are shown by the illustrations. 

 The drawings referring to the five species found in 

 the waters of the United States are reproduced in 

 figures 347 through 351. Ranson's text does not 

 include the larvae of C. gigas or 0. equestris. 



Ranson states also that the oysters can be cor- 

 rectly identified by the structure of the larval 



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