866 PELECYPODA. [Filibranchia. 



or concentrically striated, seldom ribbed, but in some species feebly 

 striated on the posterior slope. Umbones obtuse, anterior, but seldom 

 terminal. Anterior margin expanded in front of the umbones, but 

 quite smooth. Hinge-line without teeth or creriulations. Anterior 

 adductor-scar larger than in Mytilus ; posterior scars united, the byssal 

 portion long and narrow, the adductor part rounded and not bulging 

 upwards as in Mytilus. 



Distribution. About seventy species in all seas ; a few species 

 are living in fresh water. 



Fossil in the Secondary and Tertiary. 



Vernacular Name. Horse-mussel. 



Remarks. The animals of this genus have the habit of nestling 

 in a mass of byssal fibres with extraneous entangled material. In 

 some deep-water species a real nest is spun, like that of Lima, but 

 more dense. (W. H. Ball.) 



KEY TO SPECIES. 



A. Beaks almost terminal. Shell intensely black, not much winged 



posteriorly . . . . . . . . . . . . ater. 



B. Beaks a short distance from the anterior end. 



a. Shell smooth, blackish, small . . . . . . . . fluviatilis. 



1. Shell hirsute, brown, attaining a relatively large size . . nustrulis. 



1. Modiolus ater, Zelebor, 1866. Plate 58, fig. 4. 



Mytilus ater, Zelebor, Verh. Zool. Bot. Ges., Wien, xvi, 1866, 914 ; Frauen- 

 feld, Reise der " Novara," Zool., ii, Moll., 16, pi. 2, f. 30 ; M.N.Z.M., 

 167. Volsella ater, Frauenfeld : Suter, T.N.Z.I., xxiv, 223 ; Index, 94. 

 Perna confusa, Angas, P.Z.S., 1871, 21, pi. 1, f. 33. Mytilus crassus, 

 T.-Woods, P.R.S. Tas., 1876 (1877), 157. Modiola ater, Zelebor: 

 Pritchard and Gatliff, P.R.S. Vic., xvii (n.s.), 249. 



Shell small, moderately solid, smooth and shining, black, inflated, 

 with a blunt carina extending from the upper part of the umbo to the 

 posterior part of tin- basal margin. Beaks blunt, close together, very 

 near the anterior end, usually devoid of epidermis, pearly. Anterior 

 end narrowed, inflated, the, dorsal margin ascending, .straight, forming 

 a blunt angle with the posterior straight and descending portion. 

 Posterior end rounded, obtusely angled above and below, somewhat 

 compressed ; basal margin straight or concave at the anterior third. 

 Posterior part of the area concave. Sculpture consisting of concentric 

 growth-lines. Epidermis coriaceous, black, shining, turned over at 

 the margins. Colour intense black, bluish where part of the epidermis 

 has been rubbed off. Interior brownish-blue or bluish-white, slightly 

 iridescent. Margins smooth, sharp. Hinge-line triangular, smooth, 

 narrow. Ligament long and narrow, almost entirely internal. An- 

 terior adductor-scar rather long and narrow ; posterior adductor-scar 

 round and united with the long and narrow byssus-scar. Pallial line 

 distinct, slightly indented. 



