46 DUBLIN UNIVERSITY 



termediate strata. Approaching the present epoch they again become 

 abundant in the Tertiary formation, and attain their maximum forma- 

 tion in the seas of the present day. 



The following are the descriptions of two new species of Chiton from 

 the Wenlock limestone of Dudley, by Professor L. De Koninck. 



1. Chiton Gray anus {Be Koninck), PI. II., Fig. la, b, c, d. 



The dorsal cerames or intermediate plates of this species, which are 

 the only ones with which I am acquainted, are formed of two lateral 

 parts, perfectly plane, of a nearly square form, and united together by 

 an angle a little more than a right angle. The dorsal carina is most de- 

 veloped ; the anterior part of each plate is slightly crenated ; the test 

 appears to have been very thin. Their external surface is ornamented 

 by a very great number of fine parallel striae, 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 development at about the 

 middle of its growth. This interruption is indicated 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, Miinster, and C. Mempiscus, De Ryckh. It differs from 

 both, however, by the lateral margin of its plate3 being more even, by 

 the slight thickness of its test, by the absence of apophyses, and, especi- 

 ally, by the fineness and great number of striae covering its surface. 



Dimensions. — Length of the dorsal plate about 12 millimetres; 

 breadth of each side 10 mm., which gives for the complete 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 "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, 



