1883.] 279 [Hyatt. 



septa are directly continuous with the septa on the dorsal side oi 

 the living chamber, 2 and these are merely large dorsal saddles. 

 Associated species have these dorsal saddles separated as in 

 M. Barrande's typical Ascoceras. The direct derivation of Mesos- 

 ceras from Acleistoceras can hardly be doubted after comparing 

 the apertures and the forms of the short living chambers. 



Ascoceratidae. 



European Silurian forms generally have annulated whorl, long 

 living chamber, constricted near the aperture. The apertures 

 are either open or obscurely Y shaped, the two arms of the Y 

 being divided by a dorsal saddles, and the basal arm is the ven- 

 tral sinus. 



Aphragmites, Barr. Syst., Sil. Vol. 2, Text 1, p. 366, is 

 regarded by that author as a form of Ascoceras, which has re- 

 sorbed the imperfect septa in the living chamber. We, however, 

 much prefer this eminent author's first opinion, that it is a dis- 

 tinct genus with simple septa and sutures. 



Ascoceras, Barr. Syst. Sil. Vol. 2, Text. 1, p. 334, includes 

 certainly two groups, one with annulated shells, like those of 

 Aphragmites, and bearing relations to this genus similar to those 

 which Billingoceras has to Mesoceras ; and one with smoother 

 or banded and striated shells, which have apertures similar to 

 the typical Ascoceras and are in the same genus. 



G-lossoceras, Barr. ibid., p. 372, has a very slender whorl, and 

 the obscurely Y shaped apertures described above. The species 

 are Silurian, and the forms and markings of these fossils seem to 

 indicate clearly derivation from an annulated stock like the Cycl. 

 (Cyrt.) residuum sp. Barr. pi. 286, which has similar attenuated, 

 annulated whorls, but open apertures. 



Ophidioceras, Barr., includes Silurian shells closely coiled in 

 the larval and adolescent stages and open in the later stages. 

 The costated, compressed whorls have some resemblence to those 

 of Ascoceras and the aperture is closely similar to Glossoceras. 

 The shells are truly nautilian in the young and are evidently pa- 

 thological derivatives of some ancestral nautilian form. The 

 sutures are straight and the abdomen has a blunt keel. They ap- 

 pear to be the survivors of ancient ornamented series of costated 

 shells. We place them provisionally near Ascoceras on account 

 of the Y shaped apertures and form of whorl and costations. 



