446 MOLLUSCA. 
i. Long. 57, diam. 223; apert. long. 21 millim. ) 
k ” 53, ” 20; ” 21 ” 
lL , 51, 4 21; » 19 yy 
m. 4, 48, 5, 223 ” 19 ,, (specimens collected by Conradt at Panzos). 
n ” 47, bb) 19 5 9 183 99 
0. ” 36, » 17 ) ” 16 ” 
P ” 30, ” 14 > ” 13 ” 
, Two of the specimens from Panzos, n and 0, are distinctly not full-grown. It appears from these measure- 
ments that the length of the aperture is usually, but not always, a little less than the greatest diameter of the 
shell. For very young shells, see Tab. XXV. figg. 2, 3. 
Hab. S.E. Mexico: interior of the State of Tabasco, in small streams (Morelet*); 
little ridge of the Limon, State of Tabasco, in a spring (Hovirosa *?). 
N. Guatemata: Coban (Bocourt 3%); San Luis, Peten (Rockstroh, ex Stoll) ; Lake 
of Peten (Salvin 9). | 
E. Guaremana: Panzos, Polochic Valley (Conradt) ; Rio Biafra, one of the affluents 
of the Rio Motagua (v. Lhering, in litt.). 
This and the preceding variety run very much one into the other. 
Through the kindness of Canon ‘Tristram, I have been enabled to examine the 
Melaniade enumerated by him in the ‘Proceedings of the Zoological Society of 
London’ for 1863, and to satisfy myself concerning the identity of Jf. godmant with 
P. pyramidalis. 
1. Var. potamarchus. 
Pachychilus potamarchus, Pilsbry, Proc. Acad. Phil. 1892, p. 340, t. 14. fig. 7. 
Vertical plaits and spiral ridges absent, the latter wanting even on the base of the last whorl. Long. 87-99, 
diam. 29-33 ; apert. long. 32 millim. 
Hab. S.E. Mexico: Tabasco (Rovirosa 38). 
The shell figured by Pilsbry, loc. cit. t. 8. fig. 9, also wants the vertical plaits as 
well as the spiral ridges, and would seem therefore to belong to this variety; but in 
size and general form it is more like the var. pyramidalis, in company with which it 
was found. 
2. Pachychilus obeliscus. 
Melania obeliscus, Reeve, Conch. Icon. xii., Melania, t. 4. fig. 20 (1859)*; Brot, in Martini & 
Chemnitz, Syst. Conch.-Cab. ed. 2, Melania, p. 24, t. 3. fig. la’. 
Pachychilus obeliscus, Fisch. & Crosse, Miss. Scient. Mex., Moll. 11. p. 358 *. 
Somewhat intermediate between P. glaphyrus and P. lacustris, but distinct by its slender, elongate form, with 
the most prominent portion of each whorl a little above the suture, the whole shell being shaped like a 
pagoda, and resembling that of various species of Turritella, e.g. T. duplicata, L., and J. imbricata, L. 
In the typical form this prominent portion is broken up into about eight strong compressed knobs in the 
last two whorls, and the spiral ridges are here few and not very conspicuous. 
Hab. Honvuras (Dyson '). 
