332 CRETACEOUS PELECTPODA 



a little larger than the lower, and connected across the front by a distinct broadish 

 lamina ; no distinct ocelli to be observed at the edge of the mantle. 



Barbatia Helblingii, Brug., from the Mcobars. Eoot very large, strongly 

 compressed, anteriorly slightly produced, and expansile into a small disc, with 

 undulating edges, folded together when retracted ; posterior part of the foot pro- 

 vided with a very strong byssus, thin and solid at the base, expanded and fibrous 

 at the end ; gills very large, consisting of rather loose long filaments, each leaf 

 of each pair bent outwardly so as to make a free duplicature ; palps very narrow 

 throughout and long, originating near the anterior end of the gills, but distinct 

 from them, thin, almost equal, internally finely striated ; inner edge of the mantle 

 externally with a few ocelli at the antero-ventral gape of the shell. 



Sca^harca guhernaculum^ E/ceve, dredged in 2-4 fathoms near Penang. Eoot 

 compressed, strongly produced anteriorly, and expansile into a long oval disc, with 

 undulating edges, capable of being folded together, posterior part much com- 

 pressed, with a moderately thickened long byssus, of a somewhat lamellar structure ; 

 gills elongated, semioval, each leaf duplicated, and consisting of close filaments ; 

 palps large, sub-semicircular, the outer slightly larger than the inner, both inter- 

 nally striated and perfectly separated from the gills; no distinct ocelli at the 

 mantle edge. 



The form of the palps appears to be very variable, though it seems to be to a 

 certain extent constant for the different genera. In all the specimens which I saw 

 each leaf of the gills was folded in the manner above described ; I do not find a special 

 mention of this character elsewhere ; it seems, however, to be most characteristic. 



The shells always have an elongated trapezoid or sub-triangular form, closed 

 all round or gaping on the anterior part of the ventral side, with the hinge margin 

 more or less straight and provided with numerous sub-equal teeth. The genera which 

 I have here placed in the sub-family are : Cardiola, Cypricardites, BaJcewellia, Macro- 

 don, Grammatodon (sub-g. ?), Nemodon, Cucullaria, CucullcBa, Trigonoarca, Latiarca, 

 Noetia, Lunar ca, Isoarca, Argina, Senilia, Anomalocardia, Scapharca (sub-g. ?), 

 Nemoarca, Barbatia (with ^wh-g.— Cucullcearca, Acar, Galliarca, Striarca, and 

 LitharcaJ, Area. Each of these possesses some peculiarities in the form and arrange- 

 ment of the hinge-teeth, as will be seen by a comparison of their characteristics given 

 further on, but all are not sufficiently well defined, and close comparison of the fossil 

 species may greatly reduce the newly proposed generic or sub-generic divisions. 

 Deshayes, as usually, strongly objects to assail the unity of Area, but proposes 

 several sub-divisions which have a somewhat similar extent and the same object as 

 these genera. 



Area and Barbatia are already found represented in the silurian rocks, and 

 from that time the variety of forms and number of species increases until the 

 cretaceous period, where the arqinm appear to have attained the maximum of 

 development. With the cretaceous period several forms, like Macrodon, Nemodon, 

 Isoarca, and a great many others, have entirely disappeared. In the whole of the 

 tertiary period the number of generic forms and of species seems to be hardly equal 



