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the shape of Nucula, but the cardinal teeth similarly distributed as in Axincea ; 

 Limopsis has the ligament of Trigonocoelia, but the round shape of Axincea, and 

 Lyrodesma is a peculiar palaeozoic fossil with the hinge-teeth somewhat similarly 

 disposed as in Trigonocoelia; the external characters are, however, insufficiently 

 known. 



In former geological periods the axinmin^ were scarce ; they appear to have 

 been more varied as regards generic forms, but most of those remarkable types have 

 again died out, and only Limopsis, Cyrilla, and Axincea have continued up to the 

 present time. Still researches in the deep waters of tropical seas may supply many 

 of the missing links, as has only recently been proved by the discovery of Cyrilla. 

 Lyrodesma is, as already noticed, palaeozoic ; Limopsis occurs in the Trias and con- 

 tinues up to the present time ; Trigonoccelia and Nucinella are known from creta- 

 ceous and tertiary deposits; Nucunella only from the latter; the occurrence of 

 Axincea in palaeozoic rocks is somewhat uncertain, the few forms known are not 

 typical, but from the beginning of the Trias the genus occurs through all the sub- 

 sequent formations and attains its maximum of development at the present time. 



The AuciNM, as restricted, do not exhibit equally marked relations to the 

 NucuLiDm, as has been shown to be the case with the former sub-family. The animals 

 have a more elongated, geniculate foot, posteriorly often provided with a byssus, 

 which is usually solid at its base and divided into filaments at the end. It is 

 a most important organ in the economy of the animal and used for obtaining by it 

 a fixed position in crevices and hollows of rocks, sometimes exposed to the 

 heaviest attacks of the waves. The animal can, however, detach itself at will from 

 its resting place, evidently by absorbing or dissolving the terminal filaments of 

 the byssus. The byssal mass is implanted in a strong muscular tissue at the 

 posterior, generally strongly compressed, part of the foot ; it appears to be almost 

 quite solid at its base, but on close inspection the single threads can always be 

 recognised, and towards the end they are, as already stated, generally quite dis- 

 tinctly separated. Not. all the genera of amcin^, however, possess a byssus; 

 most of the species of Area and Barhatia, &c., have it largely developed ; those 

 of Scapharca apparently less, and in some species of Anomalocardia it is altogether 

 absent. Other organs appear to be equally variable. To illustrate this I shall 

 note three species which I have lately examined : 



Anomalocardia rhomhea, Born, from the Arracan coast. Poot large, roundly 

 oval, compressed, anteriorly slightly bent forward, expansile into an oval disc, 

 with undulating edges, capable of being folded together when retracted within the 

 shell; posterior part of the foot sharp with no trace of a byssus; gills large, 

 broadly sickle-shaped, filaments close together. There is a pair of gills on either 

 side, but each may be said to be again double : the basal portion is strongly 

 fleshy, and the peripherical filaments are as if bent outward on either side of 

 each leaf, so as to form a duplicature, returning almost to the base, but not grown 

 to it. Very likely these duplicatures are used for hatching eggs. Palps large, 

 elongately semioval, quite separated from the gills, internally striated, the upper 



4 M 



