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BULLETIN OF THE BUREAU OF FISHERIES. 



outer surface we find not simply one series of periostracum, prismatic, and nacreous 

 layers, but periostracum and prismatic layers, then periostracum and prismatic again, 

 and finally the nacreous layer; the outer layers arc doubled up. 



SIGNIFICANCE OF RINGS. 



In a case such as has just been described, where the outer layers are doubled up 

 as a result of an extreme retraction of the mantle, the effect of seeing a second horny 

 layer through the outer periostracum and the fairly translucent prismatic layer gives 

 the appearance of a dark band on the shell. This is the so-called growth ring, which 

 would be better termed duplication ring or interruption ring," since its significance is 

 simply that the continuity of the outer layers is interrupted and the break is repaired 

 by overlapping. In other words, the periostracum and prismatic layers are "spliced" 

 at this point. A duplication of layers should easily be observable on shells having 

 fairly light-colored or translucent periostracum but not on shells having a very dark 

 or opaque covering, and this is found to be the case. Growth rings or interruption 

 rings are commonly seen on pocketbooks, fat muckets, yellow sand-shells, floaters, and 

 other shells of fight or only medium dark colors, while they are distinguishable w'th difli- 



culty, if at all, on niggerheads, 

 piraple-backs, blue-points, and 

 other dark-colored shells. 



If the winter rings are 

 formed in the same way, and 

 the breaking of the continuity 

 of the outer layers is due to 

 the withdrawal of the mantle 

 in cold weather, then it would 

 be expected that several duplications would occur for a single winter. For cold 

 weather docs not ordinarily fall with one blow. Periods of cold and warm weather 

 alternate for a time before winter sets fairly in, and again in the spring periods of low 

 and high temperature alternate before winter is entirely passed. Such fluctuations 

 of temperature are, of course, not so frequent or noticeable in the water as in the air, 

 but they do occur. It might be expected that the mussel would react to the first sharp 

 touch of winter by closure and a sharp withdrawal of the mantle but that the deposition 

 of shell would be resumed after a time, while further interruptions and resumptions 

 of growth would occur before the full effect of winter was experienced. Again in the 

 spring there might be alternate interruptions and resumptions of growth. This, at 

 least, is the story which seems to be told by a section through a winter ring when 

 examined under the microscope. Text figure 5 shows such a section, where the alterna- 

 tion of periostracum and prismatic layers is repeated seven times, indicating six inter- 

 ruptions of growth. As virtually no increase in size occurs between the several inter- 

 ruptions, the duplicated or repeated layers are simply piled upon one another. 



Interruption rings corresponding to seasons of winter differ from those corresponding 

 to a single severe disturbance of the mussel during the normal period of growth in that 

 the latter are rings of single duplication (text fig. 4), while the former show several repe- 

 titions (text fig. 5). The winter rings in shells that have been observed are, therefore, 

 darker, though they may or may not be broader (text fig. 6). 



Fjg. s. — Section through interruption ring (winter ring) on shell of pocket- 

 book, Lampsilis venlricosa, showing repeated duplications of periostracum 

 and prismatic layers. 



o See Isely. 1914, p. iS. 



