. Bulletin. Natural history; Natuurlijke historie. 40 PEABODY MUSEUM BULLETIN 41 - •*l^^fc*k3P^*. *t*-fw t'^-' SI*'. »»»sir*»,- 32 it^lM^^BBMBi 50 Mm. o^ «MHK SOOMm. Fig. 3L Acetate peel of a vertical transverse section through the composite prismatic layer in the posterior of Spengleria rostrata. The shell exterior is toward the upper part of the photograph. The boundary between the porous acicular prismatic (above) and composite prismatic (middle) shell layers appears near the upper part of the photograph. The boundary between the composite prismatic and crossed lamellar shell layers appears
. Bulletin. Natural history; Natuurlijke historie. 40 PEABODY MUSEUM BULLETIN 41 - •*l^^fc*k3P^*. *t*-fw t'^-' SI*'. »»»sir*»,- 32 it^lM^^BBMBi 50 Mm. o^ «MHK SOOMm. Fig. 3L Acetate peel of a vertical transverse section through the composite prismatic layer in the posterior of Spengleria rostrata. The shell exterior is toward the upper part of the photograph. The boundary between the porous acicular prismatic (above) and composite prismatic (middle) shell layers appears near the upper part of the photograph. The boundary between the composite prismatic and crossed lamellar shell layers appears in the lower part of the photograph. Fig. 32. Exterior surface view of the shell posterior in Spengleria rostrata showing several first order composite prisms radiating from the umbo (upper part of photograph) toward the shell posterior (lower part of photograph). The periostracum and acicular prismatic shell layer have been removed to expose the composite prisms. tures with myostracal prismatic structures developed at sites of shell- muscle attachment. As seen on inner shell surfaces, the CL structure is generally found exterior to the pallial line, while the fine CCL structure is found mostly interior to the pallial line except in the shell posterior, where it also occurs slightly exterior to the pallial line. Figures 14 to 16 show the distribution in the shell of several varieties of CL structure defined by Carter (1976b). The first order crossed lamels in G. (R.) ovata are mostly of the branching concentric variety (BCL), while those in G. (G.) hians are of the BCL variety anteriorly and of the BCL, radiating (RCL), and blocky (BICL) varieties posteriorly. In S. rostrata RCL and BCL occur together in the shell anterior, while triangular (TCL) lamels flank BCL lamels in the shell posterior. The CL and CCL structures observed in the present species are uniformly present among the scores of individuals analyzed, but the distribution of each structure on the depositiona
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