314 Davidson — On Syringothyris. 



narrow space. This septum, which is an extension of the dental 

 lamellae, has been thickened or expanded in the inner side, as 

 shown by casts of the ventral valve, and in several specimens there 

 is a narrow semi-cylindrical depression extending nearly to the beak 

 of the valve. In comparing this with Sp. textus, we find similar 

 conditions, or more properly an extension or amplification of the 

 same features. In that species there is a convex arching pseudo- 

 deltidium, though rarely preserved in the specimens ; beneath this 

 there is a concave septum, and upon the inner face of this there is a 

 tubular callosity ; or, in other words, the inner laminae of the septum 

 become fistulose, and enclose a cylindrical or sub-cylindrical space, 

 which extends from the base of the septum to near the apex of the 

 valve. But more usually the laminae appear to be separated, and, ex- 

 tending inwards, are recurved, their edges sometimes joining to form 

 a tube, but more frequently, perhaps, the margin of each one is re- 

 curved upon itself, leaving the tube with a slit along the lower side. 

 In some instances, however, these extensions from the inner face of 

 the septum continue to the bottom of the cavity, and, joining the ex- 

 ternal shell, leave a quadrangular tube instead of a cylindrical one. 



" If, in its full development, the presence of a septum and internal 

 tube be regarded as of generic value, then we have in Sp. alta the 

 same appendages in part, or in a partially developed condition, the 

 distinct tube only being wanting. But had we the means of ex- 

 amining the internal characters of the ventral valves of all the species 

 of Spirifers, we should, probably, find graduations from the solid 

 filling of the rostral cavity, with a greater or less extension of shelly 

 matter in the form of a septum, in the fissure occupying a narrow 

 space in its apex, till we reached the development observed in S. alta 

 and S. textus, etc." It is, therefore, quite evident that much more 

 investigation with reference to the shell- structure of Spirifers, as well 

 as to their interior arrangements, will be required before they can 

 be all definitely divided into groups or genera. After a minute ex- 

 amination of several specimens of Spirifer, agreeing in shape with 

 Sp. cuspidata, and occurring at Millecent in Ireland, Dr. Carpenter 

 has found that in one example the entire shell was unquestion- 

 ably imperforate, while in others from the same locality certain 

 parts are unmistakeably perforated, although large patches remain 

 free ; and that in these last named shells, as seen in Fig. 11, the two 

 dental plates or lamellae are connected by a transverse plate, under 

 which is situated an incomplete tube, similar to the one described 

 by Professor Winchell as peculiar to his genus Syringothyris. In 

 the imperforate specimen, on the other hand, he finds the dental 

 lamellae to be unconnected by any transverse plate, and the tube of 

 Syringothyris to be altogether absent. 



The next point alluded to by Professor King has reference to 

 the Oyrtina Jieteroclita. It is quite true that in the Annals and 

 Mag. of Nat. Hist, for August, 1846 (not 1864) Professor King did 

 describe the septa and dental plates in this species, and did also 

 announce that the valves were distinctly perforated, and I much 

 regret having inadvertently omitted to state this in my Monograph, 



