14 BRITISH CRETACEOUS BRACHIOPODA. 



the apopliysary testaceous ridge, and leaving a small cavity in the upper portion of the 

 valve for the body of the animal, these sinuous grooves varying in number, position, 

 and extent, in different species ; two strong lateral adductor muscles situated under the 

 hinge. 1 



Obs. We are acquainted with only one British Cretaceous TJtecidea ; and although 

 many species are found in the various beds of this system on the continent, they have not 

 as yet been discovered in England. 



5. Thecidea Wetherellii, Morris. Plate I, figs. 15 — 26. 



Thecidea Wetherellii, Morris, 1851. Annals and Magazine of Natural History, 



pi. iv, figs. 1 — 3. 



Diagnosis. Shell inequivalve, irregular, more or less circular or pentagonal, as wide as 

 long, or longer than wide ; triangular towards the beaks, rounded laterally, and slightly 

 indented in front ; larger valve convex, attached in various ways, either by nearly the whole 

 surface of its valve, the edges only being slightly elevated, or by a small portion near the 

 beak, which is more or less produced; area narrow, deltidium large, triangular, and 

 elongated. A longitudinal depression is visible, extending along the centre of the larger 

 valve to the front, which is repeated to a lesser extent on the smaller valve ; this last 

 is slightly convex, smooth, with many concentric lines of growth ; hinge line straight, 

 articulating by means of two teeth in larger, and corresponding sockets in smaller valve. 

 In the interior of larger valve, beneath the deltidium, the short lamellar processes are seen 

 to occupy about a fifth of the length of the valve ; the central one being the longest and 

 most elevated, the other two appearing at the base of the dental plates, converge gradually 

 to near the central septum ; the inner surface of this valve is covered with close granular 

 longitudinal striae, the interior of smaller or upper valve is divided on each side in a deep 



1 In connection with the Argiope described in Part I, p. 9, Mr. Woodward and myself have re- 

 examined a suit of specimens of the recent 

 Thecidea Mediterranea, one of which (in 

 my cabinet) retains the animal in excellent 

 preservation, Thecidea has a calcareous 

 loop, folded into two or more lobes, and 

 lying in hollows of corresponding form, 

 and excavated in the substance of the 

 smaller valve ; this loop or apophysary 

 ridge supports the brachial membrane, 

 whose thickened and ciliated margin is 

 apparently attached to the inner sides of the sinuous grooves. The cilia are very long, especially the outer 

 series, which are directed inwards in the dried specimens. The loop exists in its most complete form in 

 Th. radiata (Def.) ; each lobe of the loop adheres to part of the wall of the shell along its course, 

 becoming free towards the visceral cavity. 



Th. Mediterranea. Th. radiata. 



