LOWER CRETACEOUS FORMATIONS AND FAUNAS 605 
In the lower part of the Fredericksburg division the Naticas 
and Tylostomas, in part identical with those of the Trinity con- 
tinue to play an important part. Here we find the first beds of 
Gryphaea belonging to the series of species that have often been 
grouped under the single name Gryphaea pitchert Morton, but 
which are now separated into several species. These Gryphaeas 
occur in immense numbers at intervals to the top of the Co- 
manche. Other ostreidae such as Exogyra and Alectryonia have 
a great development of individuals representing several species. 
Echinoids, belonging to the genera Enallaster, Hemiaster, 
Epiaster, Holaster, Holectypus Pseudodiadema, Cidaris, and a 
few others are also numerous and continue in greater or less 
abundance in the calcareous beds to top of the series. Among 
other common forms are Cyprimeria, with difficulty distinguish- 
able from Upper Cretaceous species, several species of Nerinea, 
Aporrhaidae, etc. Three important species of ammonites, 
Engonoceras piedernalis (von Buch), Schloenbachia acutocarinata 
(Shumard) and S. “rinitensis (Gabb) also occur. 
The Caprina limestone constituting the upper part of the 
Fredericksburg divison has an interesting and remarkable fauna 
consisting largely of Requienia, Monopleura, Ichthyosarcolites 
and other Chamidae, with Radiolites or Spheerulites, Nerinea, 
many other gastropods, corals, etc. The general assemblage 
of forms is very much like that in the ‘“Schrattenkalk” or 
‘“Caprotina limestone” of the Urgonian and the similarity 
extends to specific forms in many cases. There is aiso a more 
superficial resemblance to the Upper Cretaceous ‘ Hippurite”’ 
limestone and it was partly this resemblance that caused Roemer 
and Heilprin* to refer the Comanche series to the Upper Creta- 
ceous. While it would perhaps not be justifiable to call the 
Caprina limestone Urgonian (as defined by Choffat) or to say 
that they are exact homotaxial equivalents yet such a statement 
could not be very far from the truth. The two faunas resemble 
each other in so many particulars that there must have been free 
communication between the two areas and the conditions of 
*Proc. Acad. Nat. Sci. Phila., 1890, pp. 445-4609. 
