﻿PLATE XVII. 



I, 2. Athtris expansa, Phillips. Two remarkable malformations from the Carboni- 



ferous limestone of Hittor Hill and Longnor, in Derbyshire. 

 Collection of the School of Mines. 

 3, 4. — — A regular and typical specimen from Settle, in Yorkshire, 



collection of Mr. Burrow. 



5. — — A malformation from the same locality, from the same 



collection as 1 and 2. 



6, 7. — lamellosa, L' Eveille. From the Carboniferous limestone of Settle. 



In the collection of Mr. Burrow. These specimens are 

 remarkable, as they show that in the dorsal valve there 

 existed first a sinus, which soon became converted 

 into a mesial fold. 



8. — subtilita, Hall. From near Bolland. British Museum. 



9, 10. — — From the Carboniferous limestone of Kendal, in 



Westmoreland. 



II. ambigua, Sowerby. A very large example from Ireland. 



12. — — Interior of the dorsal valve (enlarged), from Bakewell. 



This figure has been completed from several specimens in 

 the Museum of the School of Mines. 



13. — — Ventral valve (enlarged) from silicified internal casts, from 



Bakewell in Derbyshire, and Museum of the School of 

 Mines. a, adductor or occlusor, r, divaricator, 

 muscular impressions. 



14. — — Dorsal valve of the same specimen as 13. a a, quadruple 



impression of the adductor or occlusor muscle. 



15. — globularis, Phillips. From the Carboniferous limestone of Bolland, 



and Phillips's original specimen in the British 

 Museum. 



16. 17, 18. — — From Settle in Yorkshire, collection of Mr. Burrow. 

 19. Retzia radialis, Phillips. From the original specimen in the British Museum. 



Bolland. 



20,21. — — From Settle, Yorkshire. Collection of Mr. Burrow. 



