Thomas Davidson — On Poramhonites. 53 



in different species. In P. ventricosa (Fig. 12a) they are narrow, 

 prominent, and inclose a rather wide oblong concave space, obtusely 

 angular anteriorly. In P. alta (Pander), Fig. 9, these same plates 

 are very much produced, and converge to an almost acute termina- 

 tion. Again, in P. mqiiirostris (Figs. 4 and 5), their base is very 

 wide and thickened, and they converge laterally, so as to end 

 anteriorly in an acute point. No cardinal projection, or process, is 

 observable, and the muscular impressions seem to occupy (at least 

 in part) the space inclosed by the socket plates. Two contiguous 

 primary vessels (Fig. 12a) run at once direct from the centre of the 

 valve, for some little distance, then divaricate, and finally become 

 reflected, as seen in Fig. 12a. 



Such are the principal characters observable in the interior of 

 both valves of this remarkable genus, and which I have been able to 

 study in a great number of specimens. When looking into the 

 interior of the rostral portion of both valves (as may be seen by 

 taking a squeeze in gutta-percha from that portion of the internal 

 cast), we see the hinge and the well-defined depressions or central 

 chambers circumscribed by the dental and socket plates (Figs. 9 and 

 13). "When these plates are much thickened at their base, they pro- 

 duce wide and deep indentures or depressions in the posterior 

 portion of the internal cast, as maybe noticed in Figures 3, 3a, b, c} 

 But when sharp and narrow, they leave deep slits in the same portion 

 of the internal cast, as is seen in Figures 11 and 11a. 



Porambonites therefore forms a small sub-family among the 

 Brachiopoda, attached, at least during a part of its existence, by 

 means of a pedicle, or its representative. No calcified process is 

 observable for the attachment or support of the oral appendages, 

 which were, no doubt, fleshy and spirally coiled. There are also in 

 the interior of each valve diverging dental and socket plates, which 

 laterally circumscribe the central musciilar space. 



Porambonites difi"ers materially from Pentamcrus and other genera 

 by its internal arrangements, nor is there any apparent ground why 

 it should be located in the family BJiynchonellidce. The genus is con- 

 fined to rocks of the Lower Silurian period. 



EXPLANATION OF PLATE III. 

 Fig. 1, \a, b, c. — Poramhonites cequirostris. Pander. Lower Silurian. Pawlowish, 



near St. Petersburg. 

 Fig. 2. Specimen seen from the beaks, and showing the position of 



the septa. 

 Fig. 3, 3a, b, c — Forambonites cequirostris, Pander. Internal cast. Poulkova, near 



St. Petersburg. 

 YiG. 4. — Interior of dorsal valve taken in gutta-percha from the cast 



of Fig. 3. 

 Fig. 5. Interior of the dorsal valve from a testiferous specimen. 



Lower Silurian (1« of Schmidt's classification). Kuckers, Esthonia. 



Fig. 6, 7, 8. Interior of ventral valve. Same locality. 



Fig. ^.—Porambonites alta, Pander. Interior of rostral portion of both valves to 



1 In pi. iii. figs. 28, 29, 30, of his Beitrage zur Geognosie des Eussischen reiches,, 

 Pander gives a figure of this same internal cast, but which he erroneously considers 

 to be a I'entamerus. 



