35 
g”- ; fifthly, the posterior end of the limule in J. lunulata, immediately anterior 
to the groove g^ (FI. X, Fig. 3), is far more preeipitons or straight walled than 
in I. limnana, g^\ and in consequence, when the two valves are held upright, 
side by side before the observer’s eye, there is a much greater exposure of the 
limular surface in the former species than in the latter. As further results 
of this twisting of the spiral in I. limulafa, we observe that in the luuule 
itself the groove (PI. X, Fig. 3) instead of cutting the anterior dorsal 
margin far out, does so at the anterior end of the socket 1/ (PI. X, Fig. 3) ; 
that the posterior lateral tooth is nearer the axial fulcrum in I. lunulata, and 
that the latter is much more median than in I. hnmana. 
The tightening of the coil in Fissilunula would have the following 
effect, it aj)pears to me. The cardinal tooth a (PI. X, Fig. 2) would ho 
brought more into the position of (PI. X, Fig. 3) in 1. Immlata, with an 
accompanying contraction of the supposed ligamental area m (PI. X, Fig 2), 
and if carried to excess its ultimate reduction to a deep groove only, as seen 
in the last named species (^^ PI. X, Fig. 3). Similarly the socket h (PI. X, 
Fig. 2) in Fissilunula would follow in an upward direction, and probably 
become the socket of equal alphabetical value, h^, in I. lunulata. To 
complete the analogy, the lower horizontal tooth (PI. X, Fig. 3) in the 
latter would then possibly he represented by the thickened lower margin of 
the articulus in Fissilunula, marked rP (PI. X, Phg. 2). The close analogy of 
the remaining characters, such as those of the luuule, extended anterior end, 
and posterior features of tlu' articulus, arc so obvious as not to need com])arison. 
In glancing over the more important genera of the Isocardiidm, 
such as Veniella, Glossocardia, Callocardia, Cardiodonla, l^onUicardia, 
Arctica, &c., I was certainly struck by the appearance of the hinge in the 
right valve of Feniella Conrad i, IMorton, sp., as figured by Meek,^ hut a little 
consideration will show that this resemblance is more general than intimate. 
Indeed, Avhatever likeness exists in this figure is destroyed by the very 
different aspect portrayed by AYliitfield’s illustration^ of the same valve. 
I have examined the structure of the test in Fissilunula by means of 
thin sections prepared for the microscope, and I find that the shell structure 
in question is entirely laminar, with a complete absence of tubular structure. 
This accords well with what we knoAV of that in Isocardia itself, from the 
researches of the late Dr. AV. B. Carpenter.^ 
' Report U. S. (leol. Siu v^ey Territories (Hayden’s), 1876, IX, p. 148, figs. 9 11. 
^ Mon. U. S. Geol. Survey (Powell’s), 188"), IX, t. 19, f. io. 
’ Brit. Assoc. Report for 1844 (184.5), Pt. 2, p. 102, 
