MAZATLAN BIVALVES 529 



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

 discovered among Spondylus fragments received during the 

 progress of the work.] 



[FAMILY PETEICOLID^E.] 

 GENUS 1STAEANIO, Gray. 



Arm. fy Mag. Nat. Hist. 1853, vol. xi. p. 38. B. M. Cat. Yen. 



p. 215. 



Petricola sp., auct. 

 Ohoristodon, H. fy A. Ad. Gen. vol. ii. p. 441 : non Jonas* 



680.t ? NAEANIO SCOBINA, n. s. 



? N. t. transversd, quadratd, albidd : extus creberrime granu- 

 Us in lineas diagonaliter transversis, instructis ; marginibus 

 incrementi, scepc extantibus ; umbone latd; satis prominente, 

 area lunulari magnd : dent. card. iii. quarum unus prominens 

 et alter inconspicuus parallelis ab umbone postice radiant ; alter 

 hamatus, ab altero disjunctus 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 umbo towards the posterior 

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

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

 measures long. '09, lot. *13, alt. '03. 

 Sab. Mazatlan ; 1 valve off Spondylus ; L'pool CoL 



Tablet 2516 contains the specimen. 



[FAMILY MYID^E.] 

 681. ?MYA - , sp. ind. 



Tablet 2517 contains a hinge fragment, with a deeply sunken 

 ligament-process resembling Mya. 

 Sab. Mazatlan ; extremely rare, off Spondylus ; L'pool Col. 



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

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

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

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

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

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



t tfo. 677 belongs to the Lepralia, 678 to the Cellepora, and 679 to th 

 Tubulipora, indicated in page 6. 



J The little valve deposited on tablet 781 (vide page 169, no. 220,) appears, 

 to be a different species of tite same genus 



April. 1857. xx 



