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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 convolva /w 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 Tainoceratidse 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 is shown 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* 49^> 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 costae, 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. 



