368 CEETACEOIJS PELECYPODA 



Philippi (Handb., p. 263,) already notices the animal of the type species 

 according to Miiller's figure of it. Jeffreys (Brit. Conch., II, p. 130 et seq.,) 

 describes it also, as well as that of Cr. rhombea, Berk. The former author says that 

 there is ' in front only a small opening in the mantle' for the protrusion of the 

 foot, and posteriorly a single small sessile siphonal opening. This seems to be 

 approximately correct, though the pedal opening cannot be very small. The 

 latter author states that in both species the mantle is entirely open in front, 

 ' folded behind into a sessile excurrent tube.' This last statement would appear 

 to refer to the sessile exhalant opening, which seems to be quite distinct and 

 separated from the general anterior opening of the mantle. The foot is very 

 elongated, sub-cylindrical, thickened towards the end, and expansile into a small 

 creeping disc; it very much resembles that of Lucina and allied genera. No 

 special byssus appears to exist, but the animal was observed to spin a few byssal 

 threads for its temporary attachment. 



Myoparo costatus, Lea, (Cont. GeoL, 1833, p. ip, which Conrad (in Morton's 

 Syn. org. rem. cret. form., 1834, App. p. 8,) identifies with his Stalagmium 

 margaritaceum^ is generically considered the same as Crenella, Lea's description 

 is not quite sufficient to decide that point; it notices a number of distinct hinge- 

 teeth on either side, but it does not allude to the peculiarity of the ligamental or 

 cartilage plate. Lea's figure would indicate the presence of an elongated internal j 

 pit, and in such a case the species might be generically identical with Stalagmium 

 Nystii, Galeot., as suggested by Nyst; d'Orbigny proposed (in 1850) for this last 

 species the name Nucunella (see p. 333). Good figures of these shells are a great 

 desideratum. The character of the hinge of Nucunella Nystii appears to me 

 sufficiently to indicate that the species in question belongs to the Arcidje, but if 

 Lea's species should prove to be generically the same, the name Nucunella would 

 have to be replaced by Myoparo, or Stalagmium, though I do not know whether 

 the latter can take priority before the former. Nuculocardia oi d'Orbigny is to 

 all appearance cogeneric with Crenella, 



7. JDacridium, Torell, 1859, (Bidr. till Spitzbergen's MoUuskenfauna). I 

 only know this name from Troschel's extract in Arch. f. Naturgesch., vol. xxvi, 

 p. 354). He says that the genus has been proposed for a shell considered to be 

 identical with Modiola vitrea. It is distinguished by dentibus crenulatis, ' antico 

 tuber cuUfo7'mi, postico elongato, cristis suffultifsj decurrentihus,'^ 



8. Modiolaria, Beck, 1846. Elongately oval, or sub-rhomboidal, beaks sub- 

 anterior, surface radiately striated, the striae being obsolete about the middle of 

 the ventral side, where the margin is generally somewhat indented; the striated 

 portions of the margins are crenulate ; hinge edentulous, or sometimes with one 

 or two small, sub-obsolete teeth, the marginal crenulation near the beaks is often 

 stronger than at the posterior end; type, M. discors, Linne. 



The animals of the present genus are, as already noticed, closely allied to those 

 of Crenella, but they differ essentially from those of Modiola, though the shells 

 resemble each other very much, and if in fossil species the outer striation becomes 



