MAZATLAN BIVALVES 529 



[The following species are out of tlieir order, having been 

 discovered among Spondylus fragments received during the 

 progress of the work.] 



[Family PETEICOLID^.] 



Gekus ]N"AEANI0, Gray, 



Ann. Sf Mag. Nat. Sist. 1853, vol, xi. p. 38.-5. M. Cat. Ven. 



p. 215. 

 Petricola sp., auct. 

 Choristodon, H. Sf A. Ad. Gen. vol. ii. p. 441 : non Jonas.* 



680.t ? Naeanio scobina, n. s. 



? N. t. transversa, quadratd, albidd : extus creherrime granu- 

 lis in lineas diagonaliter transversis, instncctis ; marginihus 

 incrementi, scepc extantibus ; umbone lata; satis prominente, 

 area lunulari' magna : dent. card. vi. quarum unus prominens 

 et alter inconspicuus parallelis ab umbone postice radiant; alter 

 hamatus, ah altero disjmictus marginem anteriorem continuat ; 

 nympho satis elongato. 



The great peculiarity of this shell is in the separation of the 

 cardinal margin. One tooth is formed by the twisting round 

 of the anterior margin ; the other teeth, of which one is 

 nearly obsolete, radiate from the ximbo towards the posterior 

 end; the space between being hollowed out, and as it were 

 open to the outside. Only one valve was found, t which 

 measures long. '09, lat. "13, alt. '03. 



.HaJ.— jVIazatlan ; 1 valve off Spondylus ; L'pool Col. 



Tablet 2516 contains the specimen. 



[Family MYID^.] 

 681. PMya , sp. ind. 



Tablet 2517 contains a hinge fragment, with a deeply sunken 

 ligament-process resembling Mya. 



-ffaS.— Mazatlan ; extremely rare, off Spondylus ; L'pool Col. 



* The Choristodon typicum of Jonas is the West Indian analogue of Petri- 

 cola robusta, Som. Having carefully compared a specimen from the sponge of 

 commerce with adolescent Petricola; in the Mazatlan collection, I am unable to 

 detect even a specific difference between them. The shape, minute sculpture 

 ot the valves, and other marked characters are identical. The Petricola is 

 usually seen m its adult state, with the hinge teeth nearly obliterated. 



t JNo, 677 belongs to the LepraUa, 678 to the Cellepora, and G79 to th« 

 Tubulipora, mdicated in page 6. x- j . 



^ .t 1''??J'«le valve deposited on tablet 781 (oidf page 169, no. 220.) appears. 

 tobeadifferent species of the eame genus. 



April. 1857. xx 



