14 TEREBRATULID.E. 



slight ears, or triangular expansions, at the upper angles 

 of the lower valve, as in T. caput -serpentis ; and they 

 are furnished with a very distinct and prominent crest 

 or ridge, placed inside and nearly in the middle of this 

 valve, resembling, except in position, the marginal plate 

 of Argiope cistellula. This last-mentioned character 

 likewise occurs in T. septata, Philippi, a Sicilian fossil 

 (T. septigera, Loven) , and is remarkably developed in that 

 species ; but the foramen is incomplete in T. cranium, 

 and entire in T. septata. Some specimens of T. cranium 

 have the front margin more or less truncate, and others 

 have slight and blunt ridges or angularities extending 

 lengthwise to the front margin. 



B. Shell longitudinally striate : skeleton composed of two short 

 ribs, which are looped and form a kind of ring. (Tere- 

 bratidina, D'Orbigny.) 



2. T. caput-serpen'tis *, Linne. 



Anomia caput-serpentis, Linn. Syst. Nat. ed. 12, p. 1153. T. caput-ser- 

 pentis, F. & H. ii. p. 353, pi. lvi. f. 1-4. 



Body light orange-yellow : mantle thickened by calcareous 

 spicula ; tentacles extensile and pencilled, with a crimson 

 tubercle at the base of each : arms bright orange, inclining to 

 a crimson hue ; muscular stem thick ; cirri long, ciliated all 

 over, and arranged in a single row : peduncle rather short, 

 composed of numerous loose tubular fibres. 



Shell lyre-shaped, very variable in respect of length and 

 breadth, sometimes nearly round and at other times oblong, 

 convex in the middle but compressed towards the front and 

 sides, rather solid, of a dull aspect : sculpture, scored by nu- 

 merous longitudinal striae or fine ribs, which radiate from the 

 beak to the outer margins, becoming occasionally tuberculate 

 where they are crossed by the hues of growth ; some of these 

 striae are forked, or divaricate, being simple and stronger near 

 the beak ; the surface is closely studded with microscopical 

 points, each resembling the bottom of a homoeopathic bottle, 



* From its resemblance to a snake's head. 



