Mexican Jurassic Fossils. — Johnson. i^yi 
prominent, and directed obliquely forward. Bifurcation takes 
place in some of the costae at least, two or three of them 
showing the beginning of the division just where the shell was 
broken. 
A fairly good impression of one of the medium sized Per- 
isphinctidae corresponds closely with the specimen which Cas- 
tillo and Aguilera described from Tutotepec, Distrito de Huan- 
chinango, Puebla, and compared with Perisphinctes balderus 
Oppel.* Various fragments from Catorce, San Louis Potosi, 
were also supposed to belong to this species. Our specimen 
shows the trifurcations in the last whorl mentioned by Cas- 
tillo and Aguilera, has the same (or possibly one less) nvun- 
ber of costae on this last whorl, while the costae on the first 
whorls are more marked than those in the last. In size of 
shell they agree closely. The characters of the ventre are not 
shown. Both our specimen and those of Castillo and Aguilera 
differ from the figures of Ammonites (Perisphinctes) balder- 
us Oppel by Loriolt in that the trifurcations of the costae are 
absent in the latter. 
The last specimen is a portion of the outer whorl of Per- 
isphinctes mazapilcnsis Castillo and Aguilera.:): The type spec- 
imens were described from Arroyo de los Alamitos, Sierra de 
Catorce, San Louis Potosi ; and Sierra de los Tajos o Zuloaga, 
Mazapil, Zacatecas. Our specimen shows some twelve cos- 
tae, all regularly bifurcate except two, in which the posterior 
branch of the bifurcation suffers a new bifurcation a little 
above the point of the first one. In respect to this occurrence 
the two sides of the whorl are not symmetrical, the costa on 
the one side being thus trifurcate, while its continuation on 
the opposite side is bifurcate. As a result the succeeding for- 
ward costae are no longer opposite, but alternate, the two 
branches of the bifurcated costa on the one side passing over 
the ventre, not to reunite as usual in an opposite costa, but 
one branch passing to a costa slightly behind the proper posi- 
tion of such an opposite one, while the other passes to the 
adjacent costa slightly in front of such position. This contin- 
ues until a trifurcation on the opposite side re-establishes the 
* Idem., p. 24-, lam. xi, fig. 1. 
t Atem. Soc. Paleont. Susise, vol. v. p. 94, pi. xv, fig. 7 et 8. 
t Bol. Inst. Geo]. Mex., Num. 1, p. 23, lam. x. 
