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Journal of Paleontology

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Shell Orientation Terminology among the Bivalvia (Mollusca): Problems and Proposed Solutions

¹ Department of Geology, Western Illinois University, Macomb 61455, <JB-Bailey@wiu.edu>

The first 20% of the full text of this article appears below.

TRADITIONAL TERMS used to describe shell orientation in bivalves are unsatisfactory in three important ways: 1) for poorly known bivalve taxa, unbiased and directionally neutral terms are sometimes needed to replace anterior and posterior where data are incomplete; 2) there is a striking absence of simple terms to apply to the long and short ends of the shell; and 3) the term lateral is either incorrectly or inconsistently applied. In the following discussion, I shall explore each of these issues and suggest remediation.

Although application of terminology describing shell orientation is unlikely to be an issue among extant bivalve taxa, identifying anterior and posterior ends of the shell can sometimes be a speculative business when dealing either with juveniles of living bivalves (Hoggarth, 1987) or, more commonly, with poorly studied or specialized fossil groups, especially where the internal morphology of the shell is either ambiguous, weakly impressed, or limited by preservation. The prevalence of posterior elongation among the Bivalvia has nurtured a disconcerting bias either: 1) to intuitively assume the long end of the shell to be posterior within published descriptions; or 2) to confuse right and left valves in figure captions and typological registers. In passing, it should be noted that the problem of distinguishing "head-from-tail" is not unique to the study of bivalves, but has been a significant issue in other molluscan studies, for example, sorting out the several univalve groups within the polyphyletic "Class" Monoplacophora (see Peel, 1991).

Among bivalves, beaks and umbones are rarely central in position; instead, they are placed either in front of or behind the center. Bivalves are thus described as either posteriorly elongate or anteriorly elongate, implying an a priori understanding of the orientation of the soft anatomy within the shell. Whereas the greatest diversity of bivalves . . . [Full Text of this Article]