PANAMIC-PACIFIC PELECYPODA 267 



lateral tooth. Ligament area large, wide, the nymphal plate not elevated 

 and separated from the margin by a groove. Surface color white and 

 covered by a vernicose, dehiscent periostracum. 



Key to species of Tivela of the Panamic-Pacific region 



I. Shell trigonal with high beaks, the surface slightly convex or depressed. 

 The nymphal area is a high plate separated from an expanded dorsal 

 margin by a deep slit or groove. 



A. Valve margins closed, without a posterior gap. Beaks and umbones 

 near the middle, the anterior and posterior sides of nearly the same 

 form and size. 



1. Shell large (length between SO or 60 mm.), the anterior side slightly 

 longer. Surface colored by broad rays and bands of dark brown. 



Tivela planulata- 



2. Shell smaller, equilateral. Color in lighter shades, often a cream or light 

 yellow, modified with lattice-like rays and brown zigzag markings, 

 especially on the umbones. 



Tivela plamdata undulata 



B. Posterior ventral margins of valves not closed tightly but with a small, 

 open gap. Beaks and umbones placed a little behind the middle. Shell 

 texture moderately thin, the surface depressed as if flattened. 



Tivela hians 

 II. Shell mactroid in shape and moderately convex. The nymphal area is 

 low, flattened, the bordering dorsal side not expanded. 



3. Shell large with narrow umbones, the lunular margin concave. Color 

 white and covered by a thin, dirty gray periostracum, 



Tivela argentina 



4. Shell smaller, color white or brown (if white, some brown markings 

 are often present on the dorsal slopes.) 



4a. Shell subelongated, Iphigenia-\\ke in shape, the ends somewhat 

 produced. 



Tivela delessertii 



5. Anterior and posterior ends of shell well rounded, not noticeably pro- 

 duced. Umbones full and prominent. 



Tivela byronensis 



Subgenus TIYELA s.s. 

 Tirela (Tivela) byronensis (Gray) Plate 44, figures 3, 6, 6a, 7, 8, 8a 



Cytherea radiata Sowerby, 1835, Proc. Zool. Soc. London, p. 23 (Salango and Xipixapi). — 

 Sowerby, 1851, Thes. Conch., vol. 2, p. 615. pi. 128, figs. 20-31 {Cytheraea) .— 

 Reeve, 1864, Conch. Icon., vol. 14, Cytherea, pi. 7, fig. 27. Not Trigona radiata 

 Megerle von Muhlfeld, 1811 {=Ti<vela mactroides Born, 1778). 



Trigona byronensis Gray, 1838, Analyst, vol. 8, p. 304 Pacific Ocean. 



Cytherea hindsii Hanley, 1844, Proc. Zool. Soc. London, p. 110 (Guayaquil). — Sowerby, 

 1851, Thes. Conch., vol. 2, p. 614, pi. 128, fig. 27.— Reeve, 1864, Conch. Icon., 

 vol. 14, Cytherea, pi. 9, figs. 39a, b. 



Venus solangensis d'Orbigny, 1846, Voy. Amer. Merid., vol. 5, 3rd. pt, Mollusques, 

 pp. 564, 565. (New name for C. radiata Sowerby, non radiata Chemn., 1795; 

 Risso, 1826). 



Tivela radiata (Sowerby), Romer, 1865, Monogr. Molluskengatung Venus Linne, vol. 

 1, Cytherea, 1. Sectio: Tivela, p. 13, pi. 6, figs. 1, la, lb, Ic. 



Tivela byronensis (Gray), Hertlein and Strong, 1948, Zoologica, vol. 33, pt. 4, No. 13, 

 pp. 166, 167. 



Shell of medium size, mactroid in shape but without a defined posterior- 

 dorsal area, convex, the posterior side usually a little longer and somewhat 



