. Bulletin. Natural history; Natuurlijke historie. 36 PEABODY MUSEUM BULLETIN 41 c •a. -O . 2500 _, , . , 5000 7500 age (in growth bands) at time of secretion Fig. 25. Relationship between aragonitic periostracal spike width/length ratio and age at time of secretion in Spengleria rostrata. See explanation of Figure 24. absent in adults of Recent representatives of Gastrochaena. But the pres- ence of minute spikes on shells of the Upper Cretaceous Gastrochaena (Rocellana) Linsieyi (Appendix C) and Eufistulana ripleyana (Stephenson 1941) suggests that this is an ancestral feature of the superfam
. Bulletin. Natural history; Natuurlijke historie. 36 PEABODY MUSEUM BULLETIN 41 c •a. -O . 2500 _, , . , 5000 7500 age (in growth bands) at time of secretion Fig. 25. Relationship between aragonitic periostracal spike width/length ratio and age at time of secretion in Spengleria rostrata. See explanation of Figure 24. absent in adults of Recent representatives of Gastrochaena. But the pres- ence of minute spikes on shells of the Upper Cretaceous Gastrochaena (Rocellana) Linsieyi (Appendix C) and Eufistulana ripleyana (Stephenson 1941) suggests that this is an ancestral feature of the superfamily. Aragoni- tic periostracal spikes have also been observed on the shell of a Pliocene Gastrochaena (Gastrochaena) from Florida (Fig. 30). In this fossil the spikes are restricted to the juvenile shell and were apparently not secreted by the adult. Spikelike periostracal calcification is retained in Recent gas- trochaenids only in the two surviving species oi Spengleria and perhaps in the juvenile shell oi Gastrochaena rugulosa Sowerby (USNM 184364; Pana- ma), Gastrochaena denticulata Deshayes, and certain Gastrochaena (Rocel- laria) (Carter 1976a). Most living bivalves lack distinct periostracal calcification of the kind present in 5. rostrata; so this species is a striking exception to the rule. Aller (1974) described similar spike-shaped processes cemented to the shell. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Peabody Museum of Natural History. New Haven : The Museum
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