601 
a contact zone in the later stages of development given on Pl. viii, 
Figs. 1-8. 
Another belonging to the same category is Eurystomites, of which 
the species are described and figured on Pl. v. This genus has also 
its corresponding gyroceran forms in Barrandeoceras convolvans de- 
scribed in the text and the resemblance of these to the young of 
true species of Eurystomites is very close. 
The series of the Tainoceratidz are interesting in this connection, 
because in the earliest species of Temnocheilus itself, which occur in 
the Devonian, there is no dorsal furrow, and only a contact furrow 
as isshown in Figs. 27 and 28, Pl. x. The umbilical perforation is not 
large, and in succeeding species in the Carboniferous, although 
there are several genera, there are none having the dorsal furrow. 
Metacoceras cavatiformis, Fig. 16-19, p. 496, and Pl. x, Fig. 32, 
are good examples of this group. 
Among the most remarkable of the Silurian series in which no 
dorsal furrow was present is that of the highly ornamented and 
modified genus Ophidioceras. The elaborate ornamentation of the 
shell and the costz, combined with a peculiar hollow ventral zone 
bordered by ridges, the free living chamber and aperture with 
lateral and dorsal crests show this type to be very peculiar and 
highly specialized. The small size and shape of the umbilical per- 
foration shows also very close coiling. One would suppose this 
amply sufficient in a quick-growing whorl like that of Ophidioceras 
to force the premature development of a dorsal furrow, but there is 
not the least sign of one in either of the three species examined and 
figured on Pl. viii. In this group a very interesting fact is notice- 
able in the gerontic stage. The impressed zone is persistent on the 
free dorsum until it meets a projecting spur which coincides with 
the more or less abrupt lituitean bend on the venter. On the other 
side of this spur, it has, however, such a hold upon the organization 
that it is not obliterated by the building of the spur, but is resumed 
again on the oral side, and continues to the edge of the aperture. 
In this last stage, however, the impressions made by the sharp ridges 
on the borders of the median ventral zone are obliterated, and when 
near the aperture the zone becomes narrower and shallower and 
finally disappears. 
Endolobus (Pl. viii, Figs. 36-39) is another example of the 
absence of a dorsal furrow in a good-sized umbilical perforation and 
the presence of a contact furrow in the older stages. 
PROC. AMER. PHILOS. SOC. XxxII. 143. 3X. PRINTED JULY 23, 1894. 
