52 Thomas Davidson — On Porambonites. 



measuring two inches and a quarter in length, by three inches in 

 breadth, and nearly two inches in depth. 



The valves articulate by means of two strong teeth in the ventral 

 valve, which fit into corresponding sockets in the dorsal one. 



In the interior of the ventral valve much difference is not only 

 observable in different species, but likewise so in specimens of the 

 same species, as may be seen from Figures 6, 7, and 8. The general 

 character would, however, consist in the presence of a long median 

 flattened tongue-shaped space, slightly raised near its anterior 

 angular extremity. This narrow longitudinal flattened space ex- 

 tends from under the incui'ved beak to a little in advance of half 

 the length of the valve. The dental plates, which are very large, 

 projecting, and thickened in some examples (Fig. 8), at first con- 

 verge, and afterwards run close to, and parallel with, the sides of 

 the median flattened space above described, and to within a short 

 distance of its anterior extremity. 



In other examples, such as in Fig. 6, the base of the inner sides of 

 the dental plates, or septa, as they approach the middle, become so 

 much thickened as to come into contact, leaving only a flattened 

 k)zenge-shaped area at the anterior portion of the tongue-shaped 

 space. 



Again, in some species, such as in Porambonites (Pentamerus) 

 ventricosa (Kutorga), Fig, 11, the mesial flattened space is, com- 

 paratively, wide and divided at about half its length by a narrow 

 horizontal slope. The position, and partly the shape of the dental 

 plates or septa, is sometimes seen through the transparency of the 

 shejl, or upon specimens where the shell has been worn away 

 (Fig. 2).i 



In some species, such as in P. cequirostris (Figs. 6, 7), there exists 

 a narrow raised ridge, which extends from the anterior extremity of 

 the tongue-shaped mesial space to near the middle of the frontal 

 margin, but this ridge is totally absent in other species, such as in 

 P. ventricosa. 



On the flattened surface of the anterior half of the mesial tongue- 

 shaped space are situated the adductor muscular scars, flanked by 

 the divaricator or cardinal impressions, and in this respect agreeing 

 in position with what prevails in Meristella and several other genera 

 among the Brachiopoda. The vascular impressions consist of two 

 pair of primary vessels ; one lateral pair pai'tly surround the ovarian 

 spaces, the other pair, after dividing the adductor from the di- 

 varicator muscular impressions, run to some distance towards the 

 front margin of the shell, as is seen in Fig. 11a. 



In the interior of the dorsal valve the converging socket ridges 

 form two projecting sharp or much thickened plates or septa, shorter 

 than in the ventral valve, and which sometimes show two dark 

 lines on the surface of the cast (Fig, 2). These vary considerably 



' At p. 112 of his "Monograph of the Permian Fossils of England," Prof, King 

 alludes to these plates : " Larger- valve, with two long more or less diverging dental 

 plates, and a median sinus. Small valve, with two long more or less diverging socket 

 plates." 



