27 
Order II. Pholadacea. 
Mantle with two close, more or less elongate siphonal openings 
under the hinder adductor muscle. Gills two pair, produced be- 
hind into the inhalent or lower siphon. Pedal opening generally 
small. Siphons united (except in Pharus ). 
Suborder I. Orthoconch^b. 
Body symmetrical. Valves equal, gaping at each end. Mantle 
partly exposed. Cartilage external or none. Living perpendicularly 
in holes in rocks or sand. 
A. Cartilage none, replaced by muscle. 
Fam. 1. Pholadidce. 
Foot clavate, truncated, anterior. Gills two on each side. 
t Dorsal muscle covered with shelly plates. Dwelling-cavity 
naked. Palettes none. Pholadina. 
§ Hinder gape of shell simple. 
* Front gape of the shell more or less open, never closed with an 
additional shelly plate. 
1. Dactylina. 
1. D. dactylus, t. 337. f. 4, t. 338. f. 7- 
2. Barnia. 
1. B. Candida, t. 338. f. 6. 
2. B. parva, t. 338. f. 10 (1 c. on plate). 
3. Xylophaga. 
1. X. dorsalis, t. 338. f. 4. 
** Front gape of the shell at length closed with a callous plate. 
4 . JOUANNETIA. 
1. J. globulosa, t. 338. f. 3. 
§§ Hinder gape of shell with one or two shelly or horny plates, 
forming a cup, round the siphons. 
5 . Pholadidea. 
1. P. papyracea, t. 338. f. 8. 
tt Dorsal muscle covered with a coriaceous periostraca. 
* Dwelling-cavity naked. Siphons without palettes. Zirphaiana, 
6. ZlRPHiEA. 
1. Z. crispata, t. 338. f. 5, t. 339. f. 5. 
2. Z.? Mulan, t. 338. f. 2. 
