Description of the early shell morphology of three species of Pectinidae (Mollusca: Bivalvia) from Argentina Author Oviedo, Loreley 0009-0002-3701-3692 jgloreley@gmail.com Author Pastorino, Guido 0000-0003-3341-777X gpastorino@macn.gov.ar text Zootaxa 2023 2023-11-02 5361 3 427 443 https://www.mapress.com/zt/article/download/zootaxa.5361.3.8/52200 journal article 277861 10.11646/zootaxa.5361.3.8 9df305b3-e87a-49e0-bc8a-b29ab69e66ad 1175-5326 10151183 E03BB30C-E092-4293-8ECE-CE2F402D3083 Flexopecten felipponei ( Dall, 1922 ) Figure 3 Prodissoconch The Prodissoconch-1 has a faint radial striation, without distinct commarginal growth lines, mean length is 81 µm (n = 2). The Prodissoconch-2 has thin commarginal growth lines, mean length of the P-2 is 197 µm (n = 2), and P-1/P-2 length ratio is 0.41 (n = 2) ( Table 3 , Fig. 3F ). Nepioconch The microsculpture of the LV is fine and presents regularly and densely distributed pits ( Fig. 3A, C, E ). The mean height of the LV is 1.97 mm (n = 2) and the height of the prismatic stage of RV is 1.93 mm (n = 2) ( Table 4 ). The ctenolium develops during the prismatic stage; the AA has an auricle prismatic stage. The byssal notch (RV) is deep, the byssal sinus (LV) still incipient. Post-nepioconch Ribs begin to develop rather early underlying the pitted and prismatic layer, respectively. The left valve has 28 to 34 faint folds (n = 2), with primary and secondary ribs being difficult to distinguish; there are no scales ( Fig. 3A, B, C ). The microsculpture of the fold interspaces morphs into elongate pits that, towards the shell edge, become fine antimarginal ridges and gutters ( Fig. 3C ). The posterior and anterior auricles of the LV, and the posterior auricle of the RV show similar microsculpture patterns, interrupted by prominent growth ridges. The posterior auricle is slightly longer than the anterior one (ratio AA/PA = 0.92) ( Table 4 ). The free margin of posterior auricles is straight whereas the one in the anterior auricle is anteriorly inclined. Two well defined ribs start from the ‘auricle prismatic stage’ of the anterior auricle (n = 2) ( Fig. 3D ). The byssal notch becomes sinuous and the byssal sinus is more conspicuous than in the nepioconch. The active ctenolium presents three to four teeth (n = 2) ( Table 4 ). The umbonal angle is obtuse (α =100º) (n = 2) ( Table 4 ). The shell can be white or light yellow in color.