175 
PLAGIOSTOMA., 
PECTEN, Luid. 
Gen, CHar. An oblique eared bivalve, hing’ des- 
titute of teeth or internal pit; line of the hinge 
straight in one valve, in the other gio cut 
by an angular sinus. 
Tue beak and parts about the beak of all the individuals 
of this Genus I have met with, are very thin; the sinus at 
the back forms when the valves are together a large trian- 
gular aperture, probably for the passage for the attachment 
of the animal to rocks, &c. There does not appear to be 
any true hinge. The type of this Genus has been well 
known from Luid’s time to the present day, and has had 
many Generic titles, such as Pectinites, Venus, Cockle- 
stones, &c. it is figured among the Trigonie in the Natural 
History part of the French Encyclopedia; and Parkinson, 
after he had taken much pains, thought he had found a 
tooth in the hingé, and figured it asa Donax, Org. Rem. 3. 
t. 13. f. 3. 
Tt often happens that certain species of a-Genus aré pre 
served in hard stone, or under circumstances that prevent 
our getting at those parts which are required to establish’ 
Generic characters, while others are as easily examined: 
this isexemplified in the shells of this Genus, the P. gigantea; 
although too tender to resist the changes which the thick 
valves of several of the Trigonia will withstand, is nevertlieless 
generally well preserved, but-so firmly attached internally to- 
hard stone that the inside isnot to be- got-at; whereas the 
P. spinosa often oceursin soft'chalk whieh-can'be'picked out 
