Genus MULLERIA, Fen-ussac. 



Testa irregularis, foliacea ; valde incequivalvis, cetaie 

 juniori libera, himusculosa, modioliformis ; atate 

 seniori adhcerens, unimusculosa, unilateralis ; in- 

 tUs mari/ai-iiacea. Margo cardinalis arigustiis, liga- 

 mentum partim internum. Ir/ipressio muscularis sub- 

 rotundus, posticus. Impressio pallii usque ad mar- 

 giiwm cardinalem continuus. 



Shell irregular, foliaceous ; very inequivalve, in the 

 young stage free, bimuscular, modioliform ; in 

 the adult stage attached, unimuscular, one-sided ; 

 pearly within. Hinge margin narrow, ligament 

 partly internal. Muscular impression somewhat 

 rounded, posterior. Impression of the mantle 

 continued as far as the hinge margin. 



In a very early stage this shell appears to be a re- 

 gularly formed bimuscular bivalve, resembling a Unio 

 or Modiola, with very small anterior development, 

 soon ceasing entirely. The posterior then lengthens 

 without e.xpanding, and by the union of the two valves 

 a short tube is formed, at the end of which the further 

 development and the expansion of this posterior part 

 forms a large irregular bivalve externally resembling 

 an Etheria. The absence of the anterior muscle-mark 

 is accounted for by the fact that the anterior part of j 

 the shell has never been developed after the first 

 nucleus. There seems little reason to suppose with 



some, that the muscular impressions have been united, 

 for the mantle-mark, which in bimuscular shells passes 

 li-om one muscle-mark to the other, would, in the case 

 supposed, form a loop and rectum to the posterior 

 muscle-mark, whereas it can be traced even into the 

 cavity beneath the hinge, and would probably, if not 

 obliterated, be found to reach the little anterior muscle- 

 mark in the original nucleus. 



Thus, in Etheria we have, beyond the first nucleus, 

 simply an e-xjiansion of the posterior, to the almost 

 entire extinction of the anterior side of the shell. The 

 anterior muscle becomes extinct from disuse. In the 

 preceding genus, B.\rtelettia, we have a very dispro- 

 portionate development of the hinder side with a 

 very minute anterior muscle-mark. 



Species I. (Fig. 1, a, b, c. Mus. Brit.) 



MuLLERiA LOBATA. Mid. testd transverse ovatd, vlivaceo- 

 viridi, foliaced ; valvd infciiori plerumque com- 

 jdanatd, tuhcrculis tuhiformihus armatd ; nucko 

 modioliformi, minuto, margaritaceo. 



The lobed Mulleria. Shell transversely ovate, 

 oUve-green foliaceous ; lower valve generally 

 flattened, armed with tube-formed tubercles 

 Nucleus modioliform, minute, pearly. 



Ferrussac. Mem. Mus. Hist. Nat. 



Hab. liiver Magdala, near Bogota. 



March, 1872. 



