CYRTINA. 69 



In the interior of the ventral valve two large contiguous vertical plates or septa 

 coalesce into one median plate, extending and augmenting in height from the extremity 

 of the beak to nearly the margin of the shell, and separate to form the dental plates. 

 Length and breadth very variable. One specimen measured — 



Length 24, breadth 41, depth 15 lines; but some examples have attained rather 

 larger proportions, and are not so very transverse. 



Obs. This beautiful and very interesting species has sometimes been confounded with 

 Anomites suhconicm (Martin), but with which it bears no direct resemblance. Thus the 

 last-named shell is described in the ' Petrificata Derbiensia' as differing from Sp. cuspidata 

 " in the furrows, which are few in number and acute ; in having a central angular fold 

 instead of a rounded wave in the margin ;" and it may be said to differ from Sp. sepitosa 

 in the same particulars, for in Phillips's species the ribs are very numerous, small, " and 

 divided into two, three, or four lesser ones towards the margin." There exists no acute 

 mesial fold produced by a single prominent rib, that portion of the shell presenting a 

 small, slightly convex, mesial elevation, composed of three principal ribs, but which from 

 bifurcation become six or seven as they proceed towards the margin (PI. XIV, figs, 1 and 2), 

 and it would even appear that in certain individuals the fold is itself hardly distinguishable 

 from the lateral portions of the valve. The sinus is also shallow ; and, when perfectly 

 shaped, margined on either side by a larger rib (PI. XIV, fig. 4, and PL XV, fig. 2), 

 between which may be seen a central and two smaller intercalated ones ; in other specimens 

 the sinus is composed of as many as seven (PI. XV, fig. 1). The number of ribs is like- 

 wise very variable in different examples ; thus from forty to seventy may be counted round 

 the margin of each valve of C. septosa, while seventeen only can be seen in the representa- 

 tions given by Martin of A. subconiciis ; they are likewise very variable in their respective 

 lengths and widths, which must be chiefly attributed to the intercalation of one or two 

 smaller ribs next or between those first produced (PI. XV, fig. 2). 



The most important characters presented by this remarkable shell are, however, to be 

 found in its interior arrangements, and these did not escape the notice of its first describer, 

 who, after having briefly alluded to external appearances, pointedly remarks that " the 

 septa in the lower (our ventral) valve divide it into three parts, as in Pentamerus, to which, 

 b^ this instijjicient character, it would be referred, but that many Spirifera exhibit less 

 distinctly the same phenomenon." 



Finding S. septosa to be a very rare species, but little understood or even known, the 

 exterior of the ventral valve alone having been represented in the ' Geology of Yorkshire,' 

 and those by Professor de Koninck being described under the mistaken denomination of 

 Sp. subconiciis, Martin, I did my best to assemble the few imperfect examples that were to 

 be found in our English collections,^ and to which my Belgian friend also kindly added 



^ No completely perfect adult specimens could be procured, the shell being generally found in separate 

 valves. 



