PHACOCERAS? RECTISUTURALE. Ill 



(DrumquiD), iu the county of Tyrone. Phillips records it also from the Isle of 

 Man, and M'Coy (' British Palseozoic Fossils,' fasc. iii, 1855, p. 560) from Lowick, 

 in Northumberland. 



Phacoceras ? RKCTisuTLTRALE, sp. nov. Plate XXIX, figs, la — c. 



Vescrijjtion. — Shell (cast) somewhat compressed, discoid, with a deep 

 umbilicus, in which the inner whorls must be partially if not wholly exposed ; 

 they are not visible in the (unique) specimen before me. The section of the 

 whorls in the adolescent stage is pentagonal ; expressed in more general terms it 

 may be called sagittate. In the adult, owing to the rounding off of the periphery, 

 it becomes roughly hexagonal. The periphery in the outer whorl is subacute as 

 far as the body-chamber, towards the base of which it becomes more rounded, and 

 forms a narrow ventral area. The body-chamber being imperfect, its size relative 

 to the whole shell cannot be stated. The umbilicus is very deep with steep walls, 

 the edge narrowly rounded — at least it is so in the cast. Owing to the rapid 

 lateral expansion of the whorls the sides gain in depth with corresponding 

 rapidity up to the body-chamber, near the base of which the greatest thickness of 

 the shell, viz. 45 mm., is attained, from whence it quickly tapers towards the 

 periphery (PI. XXIX, fig. 2 c). So far as can be observed the overlapping of 

 the whorls is very slight. 



The septa are moderately distant from each other in the adolescent stage, 

 their greatest width apart being here about 10 mm., but in approaching the 

 body-chamber they become crowded together, the last two being only 3 mm. 

 apart (PI. XXIX, fig. 2 a). Their sutures are remarkably straight, especially 

 near the body-chamber, indicating that the septa were extremely flat. 



The siphuncle is not seen. 



Only fragments of the test remain, and these are perfectly smooth. 



Di'mensions. 



Diameter of shell . . . .95 mm. 



„ umbilicus . . . . 28 ,, 



Height of outer whorl . . . . 43 ,, 



Thickness at umbilical margin . ' . . 45 „ 



Affinities. — In the present state of uncertainty as to the affinities of this fossil, 

 chiefly owing to the want of the inner whorls, it seems scarcely advisable to 

 attempt to compare it with any other species. I have always regarded it, 

 however, as related, though perhaps remotely, to Phacoceras oxystomum, Phill., 

 and to this, perhaps only fancied (as time may show), relationship I have given 

 practical expression by putting it in the same genus. I would, however, beg of 



