CONCHACEA. 115 



COMPLANARIA. Sw. A subgenus of Alasmodon (Unio), 

 thus described, " shell winged ; the valves connate; the bosses 

 very small and depressed ; cardinal teeth two or three ; lateral 

 teeth represented by irregular grooves. C. gigas (Unio), Sow. 

 Man. fig. 141. Alasmodon complanatus, Say) C. rugosa, Sw." 



COMPRESSED. Pressed together, or flattened. The application 

 is the same as in common use. A Patella may be described as a 

 vertically compressed cone. A Ranella, on account of the two 

 rows of varices skirting the whorls, appears, as it were, laterally 

 compressed. A bivalve shell is said to be compressed when it is 

 flat, that is, when but a small cavity is left in the deepest part 

 when the valves are closed. Perhaps the Placuna placenta, fig. 

 184, is the most remarkable instance of this. 



CONCAMERATIONS. (Con, with, camera, a chamber.) A series 

 of Chambers joining each other, as in Nautilus, Spirula, &c. 



CONCENTRIC. A term applied to the direction taken by the lines 

 of growth in spiral and other shells, (longitudinal of some 

 authors.) Every fresh layer of shelly matter forms a new circle 

 round an imaginary line, drawn through the centre of the spiral 

 cone, down from the nucleus. When the edges of the successive 

 layers are marked by any external characters, the shell is said to 

 be concentrically striated, banded, grooved, costated, &c. A fine 

 illustration of the latter is to be seen in the Scalaria or Wentle- 

 trap, fig. 351, Lines, bands, ribs, &c. in the opposite direction, 

 (transverse of some authors,) are "radiating" in bivalves, as the 

 ribs of Cardium, fig. 123, and "spiral" in univalves, that is, 

 following the direction of the whorls, as the bands of colour in 

 Pyramidella, fig. 342. 



CONCHACEA. Bl. The eighth family of the order Lamellibran- 

 chiata, Bl. The shells are described as follows : nearly always 

 regular, valves closed all round ; apices curved towards the an- 

 terior ; dorsal hinge complete, with teeth and ligament ; the 

 latter external or internal, short and thick; two distinct mus- 

 cular impressions, united at the lower part by a parallel impres- 

 sion, which is frequently sinuated at the posterior. The genera 



i 2 



