Hyatt.] 
338 
[Dec. i . 
cave one. A force applied along the center from an internal 
elastic cushion would not act upon the ends of a concave hinge 
line so as to open them out but would tend to carry them back- 
wards and closer together. The aspect of the hinge line and its 
straighter character in young shells show that it may be more 
useful in these, and generally the horny hinge is visible from the 
exterior until shells are several inches long. After this the 
valves approximate by growth, the edges fitting closely but not 
anchyiosing. They may be so close late in the life of the shell 
that the scales and ridges may appear to be continuous from one 
valve to the other, but examination shows a decided dorsal 
fissure. 
The ventral border of the shell divaricates for the accommoda- 
tion of the byssus, then approximates for a space beyond this 
posteriorly, but again opens gaping widely along the entire 
length of the ends of the valves. It is evident, when the shell is 
closed as tightly as practicable, that only the incurved parts of 
the ventral borders touch and that the posterior borders or ends 
are open and that this aperture is unprotected. 
Aviculopinna . 1 The type is the Avicula pinnaef or mis Geinitz, 2 
said to be equivalent to Pivna prisca Munst., a shell with a 
very elongated form but having a slight anterior wing. This is 
a true Aviculoid both on account of the anterior wing and also 
because, as stated by Meek, 3 his Aviculopinna americana has the 
prismatic layer which is characteristic of this group. I have 
observed this layer in A. per acuta sp. Shum., from Kidder, Mo., 
and La Salle, 111., and also in A. membranacea sp. De Koninck, 
Kildare, Ireland. 
The distribution of the nacreous layer could not be studied 
satisfactorily. It is not present at the posterior part of one 
specimen of A. peracuta for at least one half of the entire length, 
as estimated. In a stouter specimen from Ohio mentioned 
below it was present near the apex. Ko muscular impressions 
were observed nor are any figured, so far as I have seen, in this 
genus. 
A. americana of the Carboniferous has an extended posterior 
hinge and is more similar to Pteronites aliiforme Hall of the De- 
1 Silliman’s Jour. 2 ser. XXXVII, p. 212. 
2 Dyas, p. 77, pi. 14, fig. 2. 
3 Final Rept. U. S. Survey Terr. Nebraska, Hayden, 1872, p. 197. 
