IKAGIN.] 
DESCRIPTIONS OF SPECIES. 
71 
that, on the ground of the kind of rock attached to the specimen, 
Professor Steinmann regards as probably Lower Cretaceous, though 
nearly all of the Mesozoic fossils that he lists from Caracoles he con- 
siders Jurassic. I am not aware that the Chilian Ptychomya has 
been specifically named or described. 
The orbicular, toothless, divaricately ornamented shell described 
by Lycett u as Ptychomya? agassizii, and the elongate Pandora? 
cequivalvis Deshayes h and CrassateMa cornueliana d'Orb. c which have 
been referred to Ptychomya by some paleontologists, are here, follow- 
ing Stoliczka/* excluded from the genus. 
The size of the shell in Ptychomya seenis to have been less in the 
earlier history of the genus and to have reached its maximum in the 
Gault, The relation of size to time is indicated in the following table, 
in which an attempt has been made to group the species roughly in ac- 
cordance with their relative geological position, known or supposed: 
Table showing sizes of species of Ptychomya at various geological epochs. 
Species. 
Length. 
mm. 
zitteli 
90 
ragsdalei 
105 
buchiana 
120 
daedalea 
110 
(Unnamed) - 
80 
solita 
80 
(Unnamed) _ 
67 
neocomiensis 
60 
koeneni 
57 
robinaldina . 
35 
germani 
40 
complicata _ _ 
31.7 
stantoni 
32 
Geological position. 
Turonian ... ... 
Upper part of Denison stage 
Gault ... 
Aptian . : 
Aptian ... 
(? Upper part of) Lower Cretaceous 
Uppermost Neocomian (probably 
fide Toula). 
Lower and middle Neocomian . 
Neocomian e _ . . 
Lower and middle Neocomian/ 
Lower Neocomian ( Valangian) 
Lower Neocomian? (Jurassic?) .__.. 
Malone 
Locality. 
Western Alps. 
Northern Texas. 
United States of Co- 
lombia. 
Spain. 
Switzerland. 
United States of Co- 
lombia. 
Western Balkans. 
France, Switzerland. 
Argentine Cordil- 
leras. 
France, Switzerland, 
England. 
St. Croix, Switzer- 
land. 
Cape of Good Hope. 
Southwestern Texas. 
a Proc. Cotteswold Naturalists' Club, vol. 1, p. 69, PI. II, figs. 6, 6a. 
6 Mem. Soc. Geol. de France, vol. 5, p. 4, pi. 3, figs. 7a, 7b. 
c Pal. fr., Terr. Cret., vol. 3, p. 74, pi. 264, figs. 7-9. 
d See Cret. Fauna Southern India, vol. 3, p. 294. 
e Behrensden compares his species with the lower and middle Neocomian species, 
germani and robinaldina. 
f Morris and E. Forbes have recorded this species from the lower Greensand ; but 
these records are early ones (1845 and 1854) and not improbably refer to Pt neocomien- 
sis, which was not established till 1861. A species close to Pt. neocomiensis, and perhaps 
not specifically distinct from it, was referred to roMnaldina by Pictet and Renevier as 
late as 1857 to 1866. See remarks in above list of hitherto-described Ptychomya}. 
