40 CARBOMFEHOUS CEPHALOPODA OF IRELAND. 



ventral (couvex) aspect. There are thus tluee sutural regions— the horizontal, 

 tlie oblique, and the arched, which probably indicate the form of the aperture. 

 Very fine but distinct ridges or keels, perceptible to siglit and touch, traverse the 

 cast longitudinally exactly in the centre of the horizontal and arched sutural 

 regions respectivelj, and thus diametrically opposite to each other. These 

 ridges are present in all the specimens before rae wherever the removal of the 

 test permits of their being seen (PL XIII, fig. 1 c, drawn a little too broad). 

 Ridges such as these, which are met with on the casts of the shells of Ortlioceras, 

 Cyrtoceras, &c., have been called " median " or " normal " lines, but their origin 

 is unknown. 



The siphuncle is situated about its own diameter from the centre of the 

 septum, in the region nearest to the ventral border. This agrees fairl}^ well witli 

 its position as figured by de Koninck (loc. cit., pi. vi, fig. 3). It is well seen near 

 the apex in de Koninck's type specimen which I have before rae, where its 

 position does not differ materially from that which it occupies in the adult shell 

 (PL XIII, fig. 1 e). In another specimen (PL XIV, fig. 3) the siphuncle is 

 almost exactly centi-al ; it is seen at the bottom of the deeply concave septum 

 parth' indicated in the figure. Some obscure remains of the siphuncle (near the 

 upper part of the figure) seem to show that it was cylindrical, but this maj^ be 

 deceptive ; its nearly central position at least is quite clear. 



The test, which was apparently thick, is perfectly^ smooth ; it is well preserved 

 upon all the specimens before me. 



Size.— The largest uncompressed specimen, that from Oldtown (PI. XIII), has 

 a total length, excluding the apex which is broken off, of about 450 mm.; its 

 greatest diameter, measured about the middle of the body-chamber, is 73 mm. ; 

 the apical end (imperfect) measures about 18 mm. in diameter.^ 



The total length of the specimen (without body-chamber) figured by de 

 Koninck (loc. cit., pi. vi, fig. 1) is 260 mm.; the greatest diameter 70 mm., the 

 least (not far from the apex) 12 mm. 



The other specimen figured by de Koninck (pi. vi, fig. 2), though wanting the 

 body-chamber, has indications in the great depth of the last four or five chambers 

 that the latter is nearly reached, though the base of it is not seen. Probably 

 de Koninck's estimate of the total length of this specimen (500 mm.) is not 

 very wide of the mark. 



Affinities. — The only species known to me which can be compared with the 

 present one is Eusthenoceras Bailyi, de Kon., sp., about to be described. Only a 

 single specimen of it was found, but it happens to be tolerably complete, and 

 therefore admits of a comparison with E. Hulli. It consists of the greater part 



1 Though designed to show the arching of the septa and the median line, fig. 1 e also illustrates 

 the larger diameter of the fossil as contrasted with the smaller, which is seen in fig. 1 a. 



