CYRTOCERAS ARCUATOSEPTATUM. 37 



Cyrtoceras (Melocehas)' arouatoseptatum, sp. nov. Plate XI, tigs. 4, 5 a, 5 b. 



Description. — Shell of moderate size, slightly curved in the lower third, but 

 even less so above this ; somewhat compressed, probably in part by rock-pressure 

 (this species being from the cleaved rocks of the Cork district). Rate of tapering 

 1 : 4. Section elliptical, the ratio of the two diameters being as 31 : 25. Of the 

 body-chamber only a small remnant is preserved in one of the specimens before 

 me. Septa (known only by the sutures) numerous ; at a diameter of 10 mm. 

 they are 4 mm. apart., at 23 mm. they are 6 mm., and at a diameter of 32 mm. the 

 interval between them has increased to 8 mm., showing that the growth is slow 

 and gradual. In one of the specimens the sutures bend sharply upwards on one 

 aspect of the shell, with a marked obliquity on the sides whence the arches spring. 

 This distinct arching of the septa may partly arise from pressure, but whether 

 this be so or not it is prevalent throughout the specimen. The siphuncle occupies 

 a position farther from the margin than is usual in typical forms of Cijrtoceras ; 

 the nature of its elements is difficult to determine, owing to the almost total 

 destruction of all internal structures by crystallisation in the specimen cut for the 

 purpose of examining them. The vestige of the siphuncle preserved shows that 

 it was slightly inflated between the septa, and that is all that can be ascertained. 

 The test is quite smooth, and so thin and transparent that the sutures of the 

 septa are plainly seen through it. 



Size. — Length of the longest specimen 180 mm. ; greatest diameter of the 

 base of the body-chamber 55 mm. ; diameter at the (broken) apex 8 mm. 



Affinities. — The very slight curvature of this species is its most striking 

 characteristic, aud to this may be added the position of the siphuncle. These 

 features militate somewhat against the Cyrtoceran affinities of this fossil, but on 

 the whole I cannot but regard its affinities as being with the genus to which I 

 have referred it ; it seems to indicate the presence of a more generalised type 

 than the form described above under the name of G. apicale, which, however, it 

 resembles in its limited curvature. 



Remarks. — I have been fortunate enough lately to obtain another specimen 

 of this rare and interesting fossil. 1 It is less compressed than the other two 

 examples, aud therefore gives a better idea of the normal form of the species ; it 

 is imperfect at both ends, and thus can give no further information as to the 

 body-chamber or the apex. 



Attention may be drawn to the fact that fig. 4 represents the narrow aspect 

 of the shell, on which the arching of the sutures is clearly seen ; while fio\ 5 a 



1 This specimen is dow in the Dublin Museum of Science and Art. 



