316 THE VOYAGE OF H.M.S. CHALLENGER. 



which in adult shells do not reach the outer margin of the valves. Between the outer 

 ends _very short intervening ones are frequently met with, varying in number and 

 length. 



Length IG mm., height 18, diameter 4^. 



Habitat. — Stations 33 and 56, off Bermuda, in 435 and 1075 fathoms; Station 24, 

 off Culebra Island, in 390 fathoms, and off St. Thomas (depth not stated). 



This species is without difficulty separable from Amussium liicidum by its different 

 sculpture, stronger substance, thicker internal lirge, and different form of the auricles. 

 The left valve is rather like that of the above-named species in respect of sculpture, but 

 is less glossy and more strongly concentrically ridged. 



Amitssium meridionale, n. sp. (PI. XXIV. figs. 1-1 a). 



Amussinm lucidmn, var. striata, Jeffreys, Proc. Zool. Soc. LobcI., 1879, p. 562. 



Testa fragilissima, paulo insequivalvis, iridescens, pellucida, utrinque anguste hiaus. 

 Valva dextra liris filiformibus numerosis radiantibus, iucrementique lineis concentricis 

 ornata, sinistra magis opaca concentrice lirata, liris tenuissimis, complanatis, sensim 

 aecrescentibus. Auriculae subsequales, anticis interdum paulo majoribus. Pagina in- 

 terna nitida, ii-idescens, liris tenuissimis undenis instructa, in valva dextra radiatim 

 striata, striis cum liris externis congruentibus. 



Var. ; Testa liris internis duodenis muuita, valva dextra extus cancellata. 



This species is slightly oblique, excessively thin, a little inequivalve, pellucid white, 

 and narrowly gaping above on both sides. The right or deeper valve is more glossy 

 than the left, and sculptured with numerous very slender radiating lirae and concentric 

 waves and striae of growth. The other valve is ornamented with close-set very fine 

 concentric shallow grooves and depressed ridges. The former under the microscoj)e 

 exhibit a very minute reticulation, each parallel zone of this fine net-work being 

 connected with the one above and below by elongate meshes passing over the inter- 

 vening ridges. The auricles are small, at times denticulated above, slightly unequal, 

 the anterior being rather larger than the posterior, or this proportion may be reversed. 

 The beaks are moderately acute, the sides meeting at an angle of about 115 degrees. 

 The valves are glossy, somewhat iridescent within, and strengthened with eleven 

 fine lirae, the longest of which extends about two-thirds across the interior. The 

 right valve is also very finely striated, the striae corresponding to the slender lirae 

 of the outer surface. 



Length 14 mm., height 14, diameter 4. 



Habitat. — Station 158, in the Southern Ocean, south of Australia, at a depth of 



