ROPES ET AL.: GROWTH LINES OF OCEAN Ql'AHOGS 



ric growth reported for ocean quahogs by Murawski 

 et al. (1982). Light radial lines extended from the 

 umbonal area to valve margin in the periostracum of 

 the anterior half of the shell formed after growth had 

 been interrupted, but their significance was not 

 evident. 



Microstructure of Unmarked Shells 



The ocean quahog shell is entirely aragonitic with an 

 inner and outer layer separated by an extremely thin 

 prismatic pallial myostracum. The latter is com- 

 posed predominantly of irregular simple prisms 

 (ISP) and occasionally a few fibrous prisms (FP). 

 Both principal shell layers contain two growth sub- 

 layers: The thin annual growth line and the wider 

 annual growth increment. Significant variations were 

 found in the microstructure of each during 

 examinations by SEM. 



The distribution of microstructures in a typical 

 ocean quahog shell may be seen by considering a 

 transect from the exterior to interior depositional 

 surfaces. The thick, dark brown or black perios- 

 tracum is an obvious exterior surface covering, but it 

 is intimately associated with the shell. Some 

 aragonitic shell material is invariably removed when 

 peeling off the periostracum and granules of 

 aragonite were found embedded in it during 

 examination of unetched thin sections under the 

 crossed nicols of a polarizing microscope. The 

 aragonite is dissolved by etching the polished sur- 

 faces of sectioned shells, leaving cavities, some with 

 angular faces in the periostracum (Fig. 8a). 



Important microstructures for aging purposes are 

 found mostly in the outer shell layer. The dominant 

 growth increment sublayer beneath the perios- 

 tracum exhibits a granular homogeneous (HOM) 

 microstructure which is very cavernous and has 

 bleblike isolated crystal morphotypes (ICM) (Fig. 8b, 

 c). These microstructures typically grade into 

 incipient ISP (Fig. 8d). Below the prisms is a layer of 

 crossed microstructures which appear to be tran- 

 sitional between simple crossed lamellar (CL) and 

 crossed acicular (CA) structures (Fig. 8d). The latter 

 predominates in the middle portion of the outer shell 

 layer with occasional occurrences of fine complex- 

 crossed lamellar (FCCL) microstructure. Tran- 

 sitional CA-CL microstructures are also seen in 

 Figure 8e. 



In the thin growth lines of the outer shell layer, FP 

 near the external surface soon give way to very dis- 

 tinctive spherultic prisms (SphP) (Fig. 9a-d). These 

 SphP themselves grade into composite prisms 

 (CompP) which are comprised of first-order prisms 



with the second-order prisms radiating toward the 

 depositional surface from a central, longitudinal axis. 

 Closer toward the inner shell layer the FP, SphP, and 

 CompP microstructures are gradually replaced by 

 ISP bands. 



The inner shell layer is characterized by growth 

 lines composed of ISP which alternate with growth 

 increment bands of FCCL microstructures. The 

 hinge plate and tooth region have microstructures 

 recognizable as distinct sublayers that are important 

 for aging purposes. Here growth lines are construct- 

 ed of narrow ISP (Fig. 9e). These alternate with 

 growth increment bands that are composed of tran- 

 sitional CA-CL, FCCL, and HOM microstructures 

 (Fig. 9e). 



In summary, ocean quahog shells are composed 

 largely of HOM, CA-CL, and minor amounts of 

 FCCL microstructures with prismatic bands of local 

 importance. The latter constitute the growth line 

 layer; the former the growth increment layer. Figure 

 10 is a diagrammatic sketch of the distribution of 

 microstructures in the two principal layers of the 

 valve of a typical ocean quahog. 



Microstructure of Marked Shells 



Variations in the shell microstructure associated 

 with notching and subsequent shell growth of ocean 

 quahog specimens were studied by examining the 

 ventral margins of six quahogs. The same basic pat- 

 tern described for unmarked shells was observed in 

 these specimens. Optical and scanning electron 

 photomicrographs of two shells illustrate the salient 

 features (Figs. 5, 6). 



The notching event in both shells was accompanied 

 by a disruption of the normal growth pattern and a 

 resumption of shell growth at a new orientation. This 

 is seen in Figures 5a and 6a as a prominent flattened 

 surface in the exterior shell surface from the marking 

 operation followed by a lateral extension of the shell 

 margin out beyond the notch mark and old shell sur- 

 face. The extension represents renewed growth at a 

 new orientation. Retraction of the mantle during the 

 marking process and resumption of shell growth at a 

 slightly new orientation resulted in a zone of either 

 loosely calcified or uncalcified shell paralleling the 

 shell margin and extending from the notch inward 

 toward the depositional surface. This zone is filled 

 with epoxy medium during the embedding process 

 and is seen in Figures 5b and 6b, c, and d as the resis- 

 tant, unetched material penetrating several milli- 

 meters into the outer shell layer. The penetration 

 zone disappeared shortly beyond the field of view in 

 Figures 5b and 6b. Where this happened (Fig. 5c), a 



13 



