Systematic redescription of Solen (Ensisolen) tehuelchus and Ensis macha (Bivalvia: Solenoidea) from Argentina, southwestern Atlantic Ocean Author Giacomino, Sebastián 0000-0003-0253-624X Universidad Nacional de la Patagonia San Juan Bosco (UNPSJB), Puerto Madryn, Chubut, Argentina. https: // orcid. org / 0000 - 0003 - 0253 - 624 X & Instituto de Biología de Organismos Marinos (CCT CONICET-CENPAT). Bvd. Brown 2915, U 9120 ACD Puerto Madryn, Chubut, Author Signorelli, Javier H. 0000-0003-3824-0889 Instituto de Biología de Organismos Marinos (CCT CONICET-CENPAT). Bvd. Brown 2915, U 9120 ACD Puerto Madryn, Chubut, & https: // orcid. org / 0000 - 0003 - 3824 - 0889 text Zootaxa 2021 2021-04-23 4964 3 541 558 journal article 7059 10.11646/zootaxa.4964.3.6 c148b91e-4700-4337-843c-f58d7a774dff 1175-5326 4742425 5A0D449C-B9E7-46E8-B297-3572410D6407 Family Solenidae Lamarck, 1809 Diagnosis: Medium to large cylindrical shell, laterally compressed and gaping at both ends, anteriorly end very short with a sub-terminal anterior umbo. Hinge plate with two cardinal teeth, one in each valve and no lateral teeth; pallial sinus shallow, adductor muscle scars different in size and shape, the anterior muscle elongated, teardrop-like; the posterior smaller, sub-circular to oval; external and long ligament, postero-dorsally positioned ( Coan et al. 2000 ; Cosel 1990 ; Huber 2010 ). FIGURE 1. A–D: Solen ( Ensisolen ) tehuelchus MLP 3623. A–B: schematic drawings of the inside of valves. C–D: close-up view of hinge teeth. E–H: Ensis macha , MLP 6829. E–F: schematic drawings of the inside of valves. G–H: close-up view of hinge teeth. Scale bars: A–F = 2 cm; G–H = 1 cm. Comparison: In contrast to Solenidae , the family Pharidae is characterized by a medium to large elongate shell, thin, fragile, with anterior and posterior ends rounded, laterally compressed, umbo subterminal ( Siliqua , Cultellus ) to terminal ( Ensis ), external surface smooth with concentric and irregular growth lines, ligament external and opisthodetic, hinge plate with 1 to 4 cardinal teeth in the left valve, shell ultrastructure with three layers, the prismatic outer, the crossed lamellar medium and the complex cross lamellar inner layers ( Cosel 1990 , 1993 , 2009 ).