the Pholadidae into Natural Groups. 383 



** Valves with a suhcentral impressed radiating groove. Phola- 

 dopsis. 



3. J. pectinata. Pholadopsis p., Conrad, t. 39. f. 3. Tr. pul- 

 cherrima, Sow. Thes. f. 58, 59. 



6. Parapholas, Gray. Parapholas (part.), Conrad, Proc. Acad. 



N. S. Philad. 1848, 121 ; 1849, 156. 



The anterior dorsal margin reflexed, and generally furnished 

 with an internal elevated rib ; umbo with a pair of vaulted cavities 

 in front, connected with the anterior callous plate, and one or 

 two behind umbo. 



The periostraca (of this and the next genus) uniting the upper 

 and lower edge of the valves often strengthened by a shelly plate 

 forming additional elongate lanceolate posterior dorsal and ven- 

 tral valves. 



* Valves with two sunken ribs; the hinder very convex, hinder 



dorsal cavity divided. 



1. Parapholas quadi'izonalis. Ph. q., Spengler, Sow. Thes. f. 88, 



89. Ph. striatus, Chemn. fig. 864-866. Adult, Ph. Incii, 

 Sow. Thes. f. 45, 46. 



2. P. Janellii. Ph. Janellii, Desh. Ph. Californica, Conrad, Sow. 



Thes. f. 5, 6, 7. 



** Valves with a single impressed rib ; the hinder umbonal valve 

 single. 



3. P. concamerata. Ph. c, Desh., Sow. Thes. f. 67, 68. Ph. cu- 



cullata. Gray, B. M. 1840. 



7. Martesia, Leach MSS., Blainv. Mactesia, Gray, misprint 



" Mactresia, Gray," Agassiz. Penitella, Conrad, 1849. 



* Valves ivith ttvo impressed ribs, the hinder one oblique, the an- 



terior dorsal marginal reflection adpressed. 



1. M. Calva. Ph. Calva, Sow. Thes. f. 51, 52, 53. Penitella 



Wilsonii, Conrad, t. 35. f. 5, bad ? Dorsal valve less deve- 

 loped. Ph. acuminata. Sow. Thes. f. 48, 49; young. Sow. 

 Thes. f. 50. 

 1*. M. branchiata. Ph. b., Gould, Sow. Thes. f. 82, 83. 



** Valves with a single subcentral impressed rib; the anterior 

 dorsal reflection close pressed and furnished with an elevated 

 internal rib. 



2. M. ovum. Ph. ovum. Gray in Wood Supp. f. 4, Ph. ovata. 



Sow. Thes. f. 71, 72 ; dorsal bad. 



3. M. multistriata. Ph. m.. Sow. Thes. f. 35, 36. 



