320 CRETACEOUS PELECYPODA 



impression ; hinge-line crenulated or denticulated in the whole extent^ pallial line 

 entire (?) ; type, C cuUrata, Sandberger, (Rhein. Schichtensystem, &C.5 p. 276, 

 pi. 29, figs. 3 and 7). This is one of the oldest forms of nuculaninm^ differing 

 from NuGulana f== LedaJ by the internal rib-like septum below the beaks. The 

 form of the shell varies from elongated to almost quadrangularly ovate. M'^Coy 

 says that the pallial line is entire ; it may be so, but it is not clearly traceable in 

 any of the figures I saw. All the species of Cucullella are from palaeozoic rocks. 



2. Terrisonota^ Con., 1869, (Am. Journ. Conch., V, p. 98). Shell "elon- 

 gated, posterior hinge-line long, curved, linear, with numerous close transverse teeth, 

 extending nearly to the end margin ; anterior hinge area broad, oblique, and some- 

 what distant from the hinge margin ; no fosset under the apex ? " type, P. protexta. 

 Con., from cretaceous rocks of New Jersey. It would seem from the above descrip- 

 tion that the anterior part of the hinge-line is destitute of teeth, but if that should 

 not be the case, the mere form could hardly be considered as a sufficiently distinct 

 character from the next genus, at least from its sub-genus Adrana. 



3. NuGulana, Link, 1807, C= Leda,^ Scliuhm., 1817). Shell with the valves 

 closed, inequilateral, posteriorly produced and attenuated, surface finely concen- 

 trically striated and covered with a smoothish epidermis ; hinge-line divided into a 

 great number of teeth, cartilage in a small pit below the beaks, which are very 

 close to each other ; pallial line slightly sinuated ; internal nacre of shell more or 

 less distinct ; type, N. emarginata. 



3 a. Adrana, H. and A. Adams, 1857. Shell greatly elongated and thin, 

 with the valves somewhat gaping at both ends ; type, Nuc. [Adrana] lanceolata, 

 Lam. 



4. Toldia, Moller, 18y32. Shell elongated, narrowed behind, or obliquely trun- 

 cated, thin, covered with a polished olivaceous epidermis, slightly pearly within ; 

 cartilage in a large pit below the beaks ; hinge-teeth comb-like ; pallial sinus large ; 

 type, T, lanceolata, Sow. Among fossils it would seem extremely difficult to dis- 

 tinguish between Toldia and Nuculana, unless the relative size of the cartilage pit 

 and of the pallial sinus can be a guide. The animals of the former are said to 

 have no posterior appendages at the mantle below the siphons, so conspicuous in 

 NuGulana. It remains yet to be shown whether all the species of the last genus 

 really possess the same. 



4 a. Chenu adds as a sub-genus of Yoldia Morch's Fortlandia, quoting 

 (evidently by mistake) two fossil species, F. ScesendonUi, Nyst, and Y. pectinata. 

 Sow.; the former a tertiary, the latter a cretaceous shell. The first is concentrically 

 sulcated, the second radiately ribbed or striated ; both have the lunula somewhat 

 excavated, but they do not appear to me to differ from true NucuIcb, Hanley 

 (Monog. NucuLiD^J says that Fortlandia differs from Yoldia merely by having the 

 valves posteriorly closed. He quotes in the section a number of small somewhat 

 tumid species, like Y, pygmcea, Miinster. 



* This name has also been given to an Arachnoid by Koch, 



