380 Mr. J. E. Gray on the Airangement of 



XXXII. — An attempt io arrange thespecies o/^/je/cmz/yPholadidae 

 into Natural Groups. By John Edwahd Gray, Esq., F.R.S., 

 Vice-President of the Zoological Society. 



Several authors, as Lorenz Spengler (Skrivten af Naturh. 

 Gebskabet, ii. 72. 1792; iv. 40. 1798), W. Wood in his work 

 on Shells, and more lately G. B. Sowerby in his ' Thesaurus 

 Conchyliorum ' (x. 1849), have examined and described the spe- 

 cies of this family, but very few attempts have been made to ar- 

 range them into natural groups, and this is the more important 

 as the shells undergo great change in form and even in structure 

 as they are developed by the growth of the animal. 



I have therefore been induced to lay before the reader the fol- 

 lowing outline of an aiTangement which has been sometime lying 

 in my portfolio, in the hope that I might find time to prepare 

 a more complete description of the species. 



Synopsis of the Genera. 



A. Dorsal muscle protected by one or two dorsal shelly valves. 

 Cavity in which the animal lives not lined with a regular shelly 

 tube inclosing the valves. Pholadina. 



a. Siphons naked, front gape of shell open at all times. 



1. Pholas. Dorsal valves two, anterior single, central, lanceo- 



late, hinder small, transverse ; plate reflexed over the umbo, 

 closely applied. 



2. Barnia. Dorsal valve single, lanceolate ; shell ovate ; plate 



reflexed over the umbo, closely applied. 



3. Dactylina. Dorsal valves two, lanceolate, placed side by 



side ; the plate reflexed over the umbo, cellular beneath. 



4. Xylophaga. Dorsal valves two, half ovate, diverging; um- 



bonal process none. 



b. Siphons naked ; front gape at length closed, ivith a callous plate. 



5. JouANNETiA. Valvcs uucqual, left largest, in front inclosing 



the other ; dorsal valve single ; umbonal process none. 



6. Parapholas. Valves equal, regularly divided in front ; dorsal 



valves two. 



7. Martesta. Valves equal, regularly divided in front; dorsal 



valve single, lanceolate or peltate. 



c. Siphon with horny or shelly valves at their base. 



8. Pholadid^a. Front gape large, at length closed, with a 



callous plate ; dorsal valves two, small. 



