Permian Formation of Texas. 117 
could not have found a congenial habitat. That is, sandy and oth- 
er silicious strata, as has already been shown, prevail in this forma- 
tion, while calcareous strata are comparatively rare. It is true that 
certain families, especially of the Mollusca, find a silicious, sandy 
bottom, such as the material of most of those strata doubtless 
formed, more congenial than a muddy or calcareous one ; but to 
far the greater part of all invertebrate faunas the latter kind of 
bottom, other conditions being favorable, is much more congenial. 
In short the lithological character of a formation often presents ob- 
vious reasons not only for the comparative paucity of all invertebrate 
fossils in its strata, but even for the absence of representatives of 
certain families which we have every reason to suppose existed when 
they were deposited, but in other, not far distant places, and in 
But these collections, imperfect as they are, present subjects for 
consideration which are of far greater interest than that which at- 
taches to a mere addition to our knowledge of a few of the forms 
which constituted the fauna of any given epoch or period. Such, 
for example, as the relation which the fauna of one period in a given 
region bore to faunas which were presumably contemporaneous 
with it, and to those of the periods which immediately preceeded 
and followed it ; and the indication which these fossils give as to 
the geological age of the strata containing them. 
Three of the Cephalopod species, the names of which are given 
in the foregoing list, are represented on the accompanying plate, 
and brief descriptions of them follow. 
Ptychiies ciimminsi n. s. Plate I figs. 4, 5, 6, 7 and 8. 
Shell compressed-subglobose, volutions deeply embracing, umbilici 
small ; septa numerous and complex, the suture line as represented 
by fig. 8. 
Medlicottia copei n. s. Plate I, figs, i, 2 and 3. 
Shell thinly discoid, periphery narrow, medially grooved, umbilici 
small ; volutions deeply embracing ; septal suture as shown by 
fig- 3- 
Popanoceras wakotti n. s. Plate I, figs. 9, 10 and 11. 
Shell discoid ; periphery deeply embracing ; umbilici minute ; 
surface marked by slightly sinuous radiating lines or indefinite 
ridges ; septal suture as shown by fig. 11 
The other species which is definitely recognized as new is a Gon- 
iatites whose general character is not unlike that of known Carbon- 
