I 



Thomas Davidson — On Poramhonites, 51 



11. — Observations on thk GtEnus Porambonites. 



By Thomas Davidson, F.E.S., F.G.S. 



(PLATE III.) 



N 1830 Pander proposed the genus Porambonites for a remark- 

 able group of Lower Silurian Brachiopoda, which abound in 

 the neighbourhood of St. Petersburg, in Esthonia, and other parts of 

 Eussia; as well as in rocks of a similar geological age in Great 

 Britain, Portugal, and other countries. 



From the great dilficulty of obtaining well-preserved internal 

 casts, or interiors of the valves, much uncertainty has prevailed re- 

 specting its true characters, as well as with reference to the position 

 the genus should occupy in the class. Some Palaeontologists have 

 referred it to Terebratula, others to Spirifer, Orthis, Pentamerus, 

 Isorliynchus, and it has likewise been classed among the Bhyncho- 

 nellidcB by D'Orbigny and others. After many years of patient 

 search, and at different intervals, I have been able, through the 

 kindness and liberality of Dr. Volborth and Prof. F. Schmidt, of St. 

 Petersburg, to procure some good internal casts of three species, as 

 well as several detached valves and fragments showing every por- 

 tion of the interior of both valves in an admirable state of preser- 

 vation. These I now propose to briefly describe and illustrate. 



The external characters of the different species composing the 

 genus have been correctly described and figured by several palaeon- 

 tologists, and therefore all we need mention is, that the individuals 

 composing each species vary a good deal in form, and especially so 

 at different periods of their growth, as may be seen by inspecting a 

 series of examples of P. cequirostris from the dimensions of one line 

 in length to that of nearly an inch. All the species are sub-circular, 

 transversely oval, or sub-quadrate ; the dorsal valve being the 

 deepest or most convex, while there exists in the anterior half of 

 the ventral valve a depression or sinus of greater or lesser depth. 

 The beaks in both valves are so very much inflated and incurved, 

 that they generally are nearly in contact, and leave scarcely any avail- 

 able space for the passage of the peduncular fibres of attachment. 



In each valve, under the incurved beak, there exists a small narrow 

 area and triangular fissure ; the beak ridges are likewise sometimes 

 very strongly defined. In a great number of specimens the ex- 

 tremity of both beaks, and especially that belonging to the ventral 

 valve, present a circular hole. In some examples the aperture is 

 so regular in shape as almost to lead one to conjecture that it may 

 not have been entirely accidental, but it is more probable that these 

 holes have been caused by friction, due to the opening and closing 

 of the valves, the shell being very thin at that part. Externally all 

 the known species are smooth, or rather ornamented by radiating 

 rows of small pits, the shell structure, according to Dr. Carpenter, 

 being impunctate. 



The genus is composed of several well-defined species ; the largest, 

 P. gigas (F. Schmidt), PL III. Fig. 10, from Lychholm, in Esthonia, 



