194 THE VOYAGE OF H.M.S. CHALLENGER. 



the right valve is slender at first, then becomes broader below the first angle, or, in other 

 words, along the truncation, with a narrow groove along the middle which receives the 

 acute edge of the left valve. In the latter the margin is most expanded at the upper 

 angle. There does not appear to be any trace of a ligamental groove. 



Cryptodon sp. 



Habitat. — Station 344, oif Ascension Island, South Atlantic, in 420 fathoms; 

 volcanic sand. 



As but a single minute valve, only about a millimetre in length, was obtained, 1 

 refrain from giving more than a mere record of its discovery. It doubtless is the 

 young state of a probably new form approaching Cryjitodon ci'oulinensis, but rather less 

 oblique, rounder, and not so peaked at the imibones. 



Cryptodon marionensis, u. sp. (PL XIV. figs. 6— 6cf). 



Testa parva, teuuissima, alliida, subpellucida, irregulariter rotundata, mediocriter 

 convexa. Valvse impressionibus duabus radiantibus baud profuudis postice notatae, 

 incrementi lineis striatse, sculptura peculiari quasi microscopice subpunctata undique 

 ornatse. Margo dorsi ante umbones ieviter concavus, posticus vix convexus. Latus 

 anticum rotundatum, posticum baud profunde bisinuatum. Umbones parvi, acuti, 

 mediani, antrorsum versi. Cardo edentulus, sed linea cardinis in valva sinistra infra 

 apicem paulo incrassata et producta. Ligamentum omnino internum, in sulco angusto 

 infra marginem dorsalem situm. 



Length 4 mm., height 4^, diameter 3. 



Habitat. — Prince Edward and Marion Islands, in 100 to 150 fathoms. 



This species is the southern form of Cryptodon gouldii, Philippi, and Cryptodon 

 jlexuosus, Montagu, both of which species it closely resembles. It is, however, flatter, 

 and perhaps a trifle longer than either, and the lower of the two furrows, or rather 

 depressions, down the hinder side of the valves is rather broader and certainly not so 

 deep as in Cryptodon Jlexuosus. The ligament also in the present species appears to 

 be set in a somewhat deeper groove, and the prominence of the hinge-line beneath the 

 umbo in the left valve is more marked than in either of the two species referred to. 

 If these differences, slight as they are, prove constant, I think it right they should be 

 held of specific imjjortance. 



