MODIOLARIA. 125 



2. M. costula'ta *, Risso. 



Modiolus costidatus, Risso, Hist. nat. l'Eur. rnerid. iv. p. 324, f. 165. 

 Crenella costidata, F. & H. ii. p. 205, pi. xlv. f. 1. 



Shell oblong, narrower at the anterior than at the opposite 

 end, convex, rather thin, and glossy : sculpture, about 10 ribs 

 on the anterior side and twice as many on the other side ; 

 middle area somewhat depressed, destitute of ribs, but the 

 whole surface of the shell is marked by very numerous and 

 wavy transverse striae, which gives it a silky appearance : 

 colour cream-white, beautifully mottled with rich purplish- 

 brown streaks often arranged in a zigzag pattern : epidermis 

 thin, light green : margins rounded at the anterior end, nearly 

 straight or slightly incurved on the ventral side, broad and 

 rounded at the posterior end, and forming a gentle curve on 

 the dorsal side : beaks small, inflected and divergent, not so 

 much swollen as in M. marmorata, nor placed so near the 

 anterior margin : byssal sinus, ligament, and hinge-line as in 

 that species : hinge-plate strengthened by an internal rib or 

 ledge and strongly toothed ; ligamental groove narrow : hinge 

 reflected and slightly indented or notched : inside highly na- 

 creous, and of a deep purplish-brown except at the edge, 

 which is finely crenulated on the anterior and posterior sides : 

 scars indistinct. L. 0*4. B. 0-2. 



Habitat : Under stones and among small sea- weeds 

 in rock-pools at low spring-tides on the south coasts of 

 Devon and Cornwall ; Herm (Macculloch) ; Oxwich Bay, 

 near Swansea (J. G. J.) ; and Bundoran, co. Donegal 

 (Waller). It is rather local, but not uncommon. Searles 

 Wood has found it in the Red and Coralline Crag. It 

 appears to be a southern form, and to range from Brittany 

 (Mace) to Sicily (Philippi) and the Canaries (Webb and 

 Berthelot) . 



This very pretty shell differs from M. marmorata in 

 being less gibbous or tumid, much narrower, and of a 

 brighter and more variegated colour; the posterior 

 margin is regularly curved instead of obliquely wedge- 



* Slightly ribbed. 



