Placopecten Verrill, 1897
VERRILL, A. E. 1897. A study of the family Pectinidae, with a revision of the genera and subgenera. Transactions of the Connecticut Academy of Arts and Sciences, 10: 41-95, pls. 16-21. [p. 69]
«Placopecten, new subgenus of Chlamys.
Type, P. Clintonius (Say). PLATE XVII. figs. 1-7. PLATE XX. figs. 7, 8, 8a, PLATE XXI, figs. 1, la, 2, 2a,
Shell large, compressed, broadly rounded, rather thin, with simple sharp edges, meeting evenly ventrally, but gaping considerably at both ends, especially when adult (pl. xvii, fig, 5), Valves only slightly unequal in form, the right one being a little flatter, but they differ in color and somewhat in sculpture, the right one being smoother and paler. Both have fine radial lines or riblets, and they have vermiculated divergent riblets when young. Auricles small, symmetrical, nearly equal. Byssal notch small, simple. Pectinidial teeth generally obsolete, except when young. No internal ribs. Inner surface often with more or less pearly luster and a crystalline structure. Hinge-plate with two feeble, slightly divergent ribs on each end, crossed by fine transverse incisions. The foot (pl. xx, fig. 8) is well developed, oblique, slightly narrowed distally and enlarged at the end, where it is divided into two lobes by a rather deep, oblique, longitudinal fissure, so that the lobes can be spread apart or closed, at will, thus resembling somewhat the foot of Ledidae. Toward the base, on the anterior side, there is also a short, deep byssal slit, terminating at a prominent tubercle about the middle of
the front side.» ADDISON EMERY VERRILL, 1897
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Chlamys (Placopecten) Clintonius; A. E. Verrill, 1897, A study of the family Pectinidae, with a revision of the genera and subgenera, plate 17, figures 1-7.
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T. R. Waller, 2006, New Phylogenies of the Pectinidae, figure 1.2.
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«The position of Placopecten in the phylogeny of the Palliolinae is based on the demonstration of morphological transition between members of this genus and the fossil species "Chlamys" decemnaria (Conrad, 1834) by Gibson (1987: 68). At one end of its spectrum of variation "C." decemnaria is chlamydoid in form, with a complex ribbing pattern beginning ontogenetically with fine costellae that merge and cluster into coarser secondary plicae. This ribbing pattern, coarse antimarginal microsculpture between the ribs and across the disk flanks, and limited areas of "pseudoshagreen" microsculpture on the disk resemble features that are present in "Palliolum" tigerinum and higher lineages in the phylogeny (Fig. 1.2). At the other end of the spectrum of variation, "C." decemnaria approaches the shape and ribbing style of typical Placopecten, including the fossil species P. clintonius (Say, 1824), P. princepoides (Emmons, 1858), and P. sp. aff. P. magellanicus (Gibson 1987; see also Appendix Note 3). Given this transition, it is reasonable to place "Chlamys" decemnaria in the genus Placopecten (in the combination Placopecten decemnarius) and to expand the concept of Placopecten to include chlamydoid forms that show variation into Placopecten-type morphology. The reason that Placopecten in this expanded concept is placed as a sister group of the clade hat originates at Node P4 (Fig. 1.2) rather than being a member of that clade is because or differences in auricular shape and auricular sculpture. The posterior margins of the posterior auricles of Placopecten decemnarius and other species of the genus are plesiomorphically straight or convex, not concave as at Node P4, and they lack the other auricular character states mentioned above for Node P4.»
WALLER, T. R. 2006. New Phylogenies of the Pectinidae (Mollusca: Bivalvia): reconciling Morphological and Molecular Approaches. In: S.E. Shumway & G.J. Parsons (Ed.), 2006: Scallops: Biology, Ecology and Aquaculture, 1-44, figs. 1.1-1.4. [p. 19, 20]
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