460 MR. S. S. BUCKMAN ON [Nov. I903, 



length, of the body- chamber is just over half a whorl, and this last 

 half-whorl shows a tendency to excentric coiling ; there is conse- 

 quently a somewhat quick expansion of the umbilicus. The orna- 

 ment is obscure naturally ; more so by deficient preservation. In 

 the inner whorls are eostae — the important point as to whether they 

 show any nodi is not determinable. On the last whorl the costae 

 are few and distant, and tend to become obscure. 



The carina, which is small but distinct at the commencement of 

 the last whorl, degenerates to a mere ridge at last. It is presumably 

 hollow, that is a septicarina ; but the evidence is not conclusive. 

 It is set on a narrow rounded periphery. The inner margin of the 

 whorls is steeply truncate — more so in the early whorls than later. 



Affinity and Distinction. — The present species is quite 

 unlike any other with which I am acquainted. The difficulty is not 

 to separate it, but to say with what other species it can have any 

 genetic connection. My suggestion is this : — It is a platygyral 

 costate degenerative of Chartronia binodata 1 ; the inner whorls should 

 be the morphic representations of that species : the outer whorls 

 show a costate stage, which is the general rule of decline from a 

 tuberculate stage. 



History of Figured Specimen. — Found by Mr. Charles 

 Upton in the Dispansum-Bed, a portion of the Cotteswold Cepha- 

 lopod-Bed, at Buckholt Wood, near Stroud (Gloucestershire). The 

 deposit belongs to the Toarcian Stage. 



Date of Existence. — Hemera dispansi, Harpoceratan Age. 



Biological Note. 



Instances of degenerative decline (catagenesis) from the bi- 

 tuberculate to the costate stage are found in the genus Zurcheria, 

 of which the different species show the phases of a catagenetic 

 costate stage becoming more and more pronounced, while the bi- 

 tuberculate stage declines to an unituberculate stage, and is in time 

 practically lost. 2 



Similar catagenetic development may be seen in Paltopleuroceras. 

 In its acme. Paltopleuroceras may be said to be trituberculate. The 

 species show stages of decline to simple costate. 3 



Deroceras is another genus which shows catagenetic development 

 from the bituberculate to the costate stage. In many species of 

 this genus the unituberculate stage is the conspicuous feature ; in 

 other species catagenesis from the unituberculate to the costate 

 stage is shown. The unituberculate stage, however, is not directly 

 developed from a prior costate stage, but from a preceding bituber- 

 culate stage. A specimen from Lyme Begis, which is either Dero- 

 ceras armatum or a close ally thereof (PI. XXVII, figs. 5 & 6), shows 

 the bituberculate stage, and how the outer tubercle is gaining at the 

 expense of the inner one. Therefore Deroceras is derived from a 

 bituberculate form; and its ancestor is either Microderoceras Birchi, 



1 'Monogr. Inf. Ool. Aram.' pt. x, Suppl. i, p. xvi, & pi. i, figs. 11-15. (Pal. 

 Soc. vol. Hi, 1898.) 



2 See ' Monogr. Inf. Ool. Aram.' pt. vi, p 294. (Pal. Soc. vol. xlv, 1892.) 



3 See some remarks on these species in ' Descent of Sonninia & Harrwnato- 

 ceras] Quart. Journ. Geol. Soc. vol. xlv (1889) pp. 653, 654, under Pleuroceras. 



