365 



MONOGEAPHS OF THE GENEKA 



ORBICULA, CRANIA, HIPPONYX 



AND 



THECIDIUM. 



ORBICULA. 



Testa suborbicularis, insequivalvis, compressa, valvS, superiore 

 depresso-conica, vertice subceiitrali ; valva inferiore extils disco 

 parvo, postico, pediculi adhsesione^ processu obtusiusculo intus 

 terminato ; cardine ligamentoque cardinis nullis. 



A nearly orbicular, rather irregular, inequivalve, compressed 

 bivalve, affixed by a tendinous pedicle, which spreads over a small 

 disk placed near the posterior part of the lower valve externally ; 

 in the middle of tliis disk is a fissure, which divides it longitu- 

 dinally, which opens into the valve and is terminated anteriorly by 

 an obtuse process ; the upper valve is of a flattened conical form 

 with a nearly central vertex. The Orbiculse consist almost entirely 

 of a horny substance, with very little, if any, t-estaceous matter. 



1. Orbicula lamellosa (pi. Ixxiii. f, 1.) Brod. Zool. Proc. 

 1833. p. 124. 



Testa cornea, fusc^, suborbiculari, subdepress^, lamellis concen- 

 tricis, elevatis, rugosa. 



Shell of a horny substance, chesnut brown, nearly orbicular, rather 

 depressed, covered with elevated concentric lamellae ; disk of adhe- 

 sion oblong, close to the posterior edge, perforation oblong, placed 

 near the hinder part of the disk. 



Found in groups on various parts of the coasts of Peru, 

 H. Cuming. 



2. Orbicula l^vis (pi. Ixxiii. f. 2, 3.) Sow. Trans. Linn. 

 See. vol. xiii. 



Test6, Isevi, tenuiuscul^, apice ex margine remote. 



Shell smooth, rather thin ; apex distant from the margin ; disk 

 of adhesion oblong, not close to the posterior margin ; perforation 

 short, near the hinder edge of the disk of adhesion. 



