72 CARBOiNIFEROUS CEPHALOPODA OF IRELAND. 



more rectangular in A. trochlea, which naturally results from the greater diver- 

 gency of the sides in the latter species just referred to, 

 Localiti/. — Cookstown, county of Tyrone. 



Mksoohasmoceras/ gen. nov. 



The first whorl forms a very wide curve, leaving an unusually large central 

 vacuity which is the most striking feature of this genus, none other approaching 

 it in this particular. The shell is throughout greatly compressed, as shown in 

 the ratio of the two diameters of the section, the height being to the thickness 

 respectively as 20 : 12. A third characteristic of great importance is the 

 complete absence of ornamentation at all stages of growth. This differentiates it 

 from Apheleceras, which it at first sight resembles, and to which it is apparently 

 allied. 



The young shell is nearly ovate in section, but before the first whorl is reached 

 it becomes angular on the peripheral and umbilical borders. Before the com- 

 pletion of the first whorl the periphery gets flattened and the sides compressed 

 (cf. PI. XX, fig. 15 d), an imperfectly defined lateral angle being developed, 

 which persists throughout the growth of the shell, but never becomes sharply 

 defined. The overlapping of the whorls is so slight as not to extend beyond the 

 borders of the periphery. 



The septal characters are but imperfectly known, my material not being 

 sufficient to enable me to make a complete study of them (PI. XX, fig. 14 h). 



The siphuncle is seen only in section (PI. XX, figs. 15 c, d). 



It will be useful here to tabulate the principal characters of Mesochasmoceras, 

 and to place in contrast with them those of Apheleceras (PI. XXI, figs. 1 — 7), 

 thus : 



Ifesochasmoceras. Apheleceras. 



Whorls greatly compressed. Whorls compressed. 



Periphery a little narrower than the sides ; Periphery extremely narrow ; deeply chan- 



slightly channelled. nelled. 



Umbilical vacuity excessively large. Umbilical vacuity moderately large. 



Test unornamented throughout. Test ornamented in the young stage. 



Ratio of height to thickness of whorl, respec- Eatio of height to thickness of whorl, respec- 

 tively as 20 : 12. tivel y as 25 : 20. 



The genus Mesochasmoceras falls under Hyatt's family Trigonoceratidse, which 

 includes the following genera : Trigonoceras, M'Coy ; Coelonautilus, Foord, 



' From /ieo-fj^fia/ia , au opening in the middle ; Kepas, a horn. 



