ACTINOCERAS GIGANTEUM. 29 



1880. Orthoceras giganteum, L. G. de Konwck. Faune Calc. Curb. Belg., 



torn, v, p. 75, pi. xliv, figs. 5 — 10. 

 1888. Actinoceras giganteum, A. H. Foord. Cat. Foss. Cepl). British Museum, 



pt. 1, p. 1S7. 1 



Description. — Shell very large, straight. Section nearly circular. Rate of 

 increase varying from 1 : 7 to 1 : 5. Septa moderately convex, somewhat oblique 

 in the upper part of the shell, increasing somewhat rapidly in their distance 

 apart; for example, in a specimen measuring 850 mm. in length the septa are 

 only 2 mm. (about) distant at the smaller extremity, while at the larger the interval 

 between them has increased to 25 mm. There is a large fragment contained in 

 the Museum of Science and Art, Dublin, from the red limestone of Castle Espie, 

 county of Down, which has the following dimensions : length 750 mm., greatest 

 diameter 280 mm., least 155 mm. Another specimen, collected by myself at 

 Clane, in the county of Kildare, has a length of 910 mm., the greatest diameter 

 being 240 mm., the least 35 mm. ; this gives a rate of tapering of about 1 : 4*5. 

 The distance of the last three septa from each other is from 40 to 50 mm., that of 

 those at the smaller end (where the diameter is 55 mm.) from 11 to 12 mm. The 

 length of the portion of the body-chamber preserved is 240 mm. The specimen 

 is chemically eroded, the surface roughened and destroyed, and showing no trace 

 of the test. The siphimcle is not large in proportion to the size of the shell ; it is 

 a little excentric in position in the young shell, tending to become more nearly 

 central in the adult ; it is much inflated between the septa, where it forms 

 depressed spheroidal segments twice as wide as long. The outer surface of the 

 test is generally wanting, and the inner layer being quite smooth has led to the 

 species being described as having a perfectly smooth shell. There is a specimen, 

 however, in the British Museum which has a distinctly striated surface above the 

 smooth layer, the ornaments consisting of fine transverse lines, of which there 

 are about three in the space of 2 mm. 2 



Affinities. — The only species with which the present one may be compared is 

 that next to be described, and the characters separating the two species being 

 enumerated under the latter, it is not needful to mention them here. 



Remarks. — Of the records of this species in foreign localities of the Carbo- 

 niferous rocks only two need be referred to, viz. that of de Koninck and that of 

 Roemer. The former of these authors has described and figured the species in 

 his well-known work on the Carboniferous fauna of Belgium, where it occurs in 

 several localities. The latter has figured and very briefly described a fragment 

 from the Kulmkalk of Grund, in the Hartz Mountains (' Palasontographica,' 1854, 



1 The list here supplied gives only the more important synonyms and references ; it does not 

 claim to be exhaustive. 



2 The specimen showing these markings is registered C 325 in the collection. 



