from the Upper Silurian ( Wenlock Limestone i of Dudley, 97 



thin. Their external surface is ornamented by a very great num- 

 ber of fine parallel stria? or lines of growth ; on the lateral and 

 anterior sides of each plate, and between them, there are ex- 

 tremely thin ribs covered with small granulations. Each of 

 these plates appears to have undergone a suspension of develop- 

 ment at about the middle of its growth; this interruption is indi- 

 cated by a striation much larger and deeper than the others, which 

 are all nearly equal in strength. The median and lateral areas are 

 very nearly equal, and divide each side of the plate into two parts. 



It is probable that, if this species was furnished with 

 apophyses, they were very small, as I have not been able to 

 discover any trace of them on the various specimens I had the 

 opportunity of examining. 



Relations and Differences. — This Chiton presents a greater 

 similarity with C. priscus, Munster, and C. Mempiscus, De Ryckh. 

 It differs from both, however, by the lateral margin of its plates 

 being more even, by the slight thickness of its test, by the ab- 

 sence of apophyses, and especially by the fineness and great 

 number of stria? covering its surface. 



Dimensions. — Length of the dorsal plate about 12 milli- 

 metres ; breadth of each side 10 mm., which gives for the com- 

 plete animal an approximate length of from 80 to 90 millimetres, 

 and a mean breadth of 16 to 18 mm. 



Locality, — This species has been discovered by Messrs. Gray 

 and Fletcher in the Upper Silurian ( Wenlock limestone/ near 

 Dudley. 



2. Chiton Wrightianus, De Koninck. (PI. II. fig. 2 a, b, c.) 



The form of the dorsal plates of this species is subtriangular, 

 the posterior edges making very nearly a right angle. The 

 lateral angles are rounded, and the anterior edge is very sinuous. 

 All the plates are supplied with a well-marked median carina, 

 and appear to have been without apophyses. The surface is 

 covered with a small number of deep equidistant stria?. The test 

 is slender. The median area is larger than the lateral one. 



Relations and Differences. — This Chiton very much resembles 

 C. Loftusianus, King, but differs from it in the regularity of the 

 stria? of the median and lateral areas, and by the more marked 

 sinuosity of the anterior edge of its plates. 



Dimensions. — The length of each dorsal plate is about 8 milli- 

 metres, and the breadth 12 mm. 



Locality. — This species was found by Mr. Gray with the pre- 

 ceding one ; it is, however, scarcer than even that. 



EXPLANATION OF PLATE II. 



Fig. 1 a, Chiton Grayanus, De Kon., nat. size, with fragments of four 

 Ann. fy Mag. N. Hist. Ser. 3. Vol.vi. 7 



