﻿NORWOOD AND PRATTEN ON PRODUCTI. 15 



extends inwards to the centre of the valve. On each side of this are the hepatic 

 apophyses, small and triangular; by the sides of these are two depressions, and pro- 

 jecting forwards and laterally from these are two large callosities, bounded, anteriorly, 

 by a prominent ridge. On the anterior portion of the visceral disk are several long, 

 obtuse points, inclining forwards, posterior to which the surface is covered by other 

 shorter ones. In front, and along the sides of the visceral disk, are a great number 

 of points imbedded in the shell and looking forwards, but few of which rise above its 

 surface. From near the extremity of the tooth, at a short distance from the cardinal 

 border, proceeds, on each side, a strong and rather sharp ridge, running towards the 

 lateral angles of the shell, but increasing its distance from it as it recedes from the 

 tooth. Just before reaching the lateral edges it changes its direction, bends around 

 with them, and becomes lost, being absorbed by the shell. This ridge, with the front 

 of the shell, presents an oval outline. 



Dimensions. — Length, 14 millimetres; breadth, 19 mil. Proportions. — Length, 

 100; breadth, 135; thickness, 0.42 4 



Ribs. — The whole number is from 22 to 24. 



Comparisons and Differences. — By the small number of ribs, the rugose appearance 

 of the visceral disk, and the great number of slender spines, with which its entire 

 surface is garnished, this Productus may be easily distinguished from any other known 

 species. 



' Geological Position and Localities. — This fossil belongs to the coal measures, and is 

 found in the calcareous clays of that formation. It occurs at Fish-hook creek, Pike 

 county, Illinois, and six miles north-west of Richmond, Missouri. 



Explanation of the Figures. — PL I., Jig. 8, a. View of the dorsal valve of an adult 

 individual. 



Fig. 8, b. Ventral aspect of the same. 



Fig. 8, c. The same shown in profile. 



Fig. 8, d. View of the interior of the ventral valve, showing the tooth, the median 

 ridge, 'the hepatic apophyses, the callosities, and the lateral ridges. 



Fig. 8, e. View of the interior of the dorsal valve. Illinois State Collection. 



P. Portlockianus, nob. — PI. I., fig. 9, a, b, c. 



Shell of a medium size; transverse; dorsal valve gibbous, much enrolled on itself, 

 and divided by a wide deep sinus, originating at a short distance from the beak, and 

 extending to the anterior border. Beak thick and short, not passing the cardinal 

 border. Cardinal border longer than the transverse diameter of the shell. Ears 

 rather long, rounded; follow, in a great degree, the vaulting of the dorsal valve; 

 covered with thick transverse folds; two or three tubes on each. Visceral disk large; 

 a little flattened ; crossed by very decided transverse folds, twice as large as the ribs 



