Centimeters 



[CURE 33. — Principal axis of growth of a C. virginica 

 from Chatham, Mass., is deeply marked by a pigmented 

 band. 



has shifted into the opposite direction (fip;. 34). 

 The "norniid" oyster (the right side of tlie figure) 

 is curved to the left while in the inverted specimen, 

 shown on the left of the figure, the shell curves 

 to the right. Such "right-handed" oj'sters are 

 probably common in all oyster populations since 

 they were found in Texas, Chesapeake Bay, 

 Narragansett Baj', and Great Bay, X.H. In 

 every other respect the inverted specimens are 

 normal and had typicaUv cupped left valves with 

 well-developed grooved beaks. There is no evi- 

 dence that inversion was caused by jnechanical 

 obstruction or some unusual position on the 

 bottom. 



Complete inversion in bivalves was described 

 by Lamy (1917) for Lncina, Chnma. and several 

 species of the subgenus Goodallia (fain. Astartidae). 

 It consisted in the appearance of structures, typical 

 for the right valve, on the left valve and vice 

 versa. In the case of C. inrginica the structural 

 elements remain unaffected and the inversion is 

 limited to the contours of the valves. 



MORPHOLOGY AND STRUCTURE OF SHELL 

 733-851 0—64 3 



The once established principal axis of growth 

 does not always remain unchanged. Occasionally 

 old oysters are found in which the direction of 

 growth had undergone sudden changes of about 

 90°. The change shown in figure 35 took place 

 when both oysters were about 6 to 7 years old. 



The instability of the principal axis of growth 

 may be even more pronounced. My collection 

 has an oyster {C. inrginica) found on the banks of 

 a lagoon near Galveston, Tex., in which the princi- 

 pal axis, clearly indicated by pigmented bands on 

 the surface of the valves, changed its direction at 

 the end of each growing period. The resulting 

 zigzag line is clearly visible in the specimen (fig. 

 36). 



DIMENSIONAL RELATIONSHIPS OF 

 SHELL 



Shape of a bivalve sliell is often expressed as 

 a ratio between its height and length or by some 

 other numerical index. Lison (1942) pointed out 

 that the shape of an oyster shell cannot be ex- 

 pressed in precise geometrical terms, presumably 

 because of its great variability. The "index of 

 shape" determined as a ratio of the sum of height 

 and width of a shell to its length was used by 

 Crozier (1914) in studying the shells of a clam, 

 Dosrnia discus. For the moUusks ranging from 2 

 to 7 cm. in length collected near Beaufort, N. C. 

 this index varied from 1.24 to 1.28 indicating that 

 the increase of the species in height and width was 

 directly proportional to the increase in length. 

 Such regularity is not found in tlie sliells of adult 

 C. inrginica taken at random from commercially 

 exploited bottoms. For tlie entire range of 

 distribution of this species in the Atlantic and 

 Gulf states the index of shape varied from 0.5 to 

 1.3. The histogram (columns in figure 37) shows 

 nearly normal frequency distribution with the 

 peak of frequencies at 0.9. No significant dif- 

 ferences were found in the index of shape in the 

 nortliern and southern populations of oysters 

 examined separately. Tlie boundary between the 

 two groups was arbitrarily drawn at the Virginia- 

 North Carolina line. The two cm'ves connecting 

 the frequency points on figure 37 indicate that in 

 the southern population the index of shape ex- 

 tends from 0.5 to 1.3, while in the northern oysters 

 it varies from 0.6 to 1 .2. The difference is probably 

 not very significant, but it may be due to a greater 

 percentage of wild oysters on commercially ex- 

 ploited natural bottoms of the southern states. 



29 



