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PLAGIOSTOMA. 
PECTEN, Luid. 
Gen. Char. An oblique eared bivalve, hinge des- 
titute of teeth or internal pit; line of the hinge 
straight in one valve, in the other deeply cut 
bj an angular sinus. 
The 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 Trigoniae in the Natural 
History part of the French Encyclopaedia ; and Parkinson, 
after he had taken much pains, thought he had found a 
tooth in the hinge, and figured it as a Donax, Org\ Rem . 3 . 
t. 13. f. 3. 
It often happens that certain species of a Genus are 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 is exemplified 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 nevertheless 
generally well preserved, but so firmly attached internally to 
hard stone that the inside is not to be got at, whereas the 
P. spinosa often occurs in soft chalk which can be picked out 
