PURPTJRA. 159 



TRIBULUS. Spire depressed, whorls simple, the last ventricose ; aperture 

 wide ; columella arcuated ; inner lip excavated, corrugated at the fore- 

 part. 



THALKSSA. Spire elevated, whorls spinose, augulated at the upper part ; 

 aperture moderate ; columella rounded, tubercular in front ; outer lip 

 nodulous internally. 



STRAMONITA. Spire elevated, whorls simple or nodulous ; aperture 

 moderate, produced anteriorly ; columella rounded, simple in front. 



TROCHIA. Whorls separated by a deep groove ; inner lip thickened, 

 convex, striated ; aperture with a very short canal. 



POLYTBOPA. Spire acuminate, whorls foliated or tuberculose ; inner lip 

 flattened ; canal small, oblique ; aperture narrowed at the fore-part. 



CRONIA. Shell ovate ; spire acuminated ; aperture moderate ; inner lip 

 callous at the upper part ; columella straight, simple anteriorly. 



Genus PURPURA, Brug. 



For the dentition of P. patula, L., see PI. 42, fig. 1. The 

 same plate contains figures of the lingual armature of represen- 

 tatives of the different subgeneric groups. 



The animal does not differ essentially from that of Murex in 

 its general external and anatomical characters. The eyes un- 

 usually placed near the tips of the tentacles, the siphon is short, 

 and the foot not large. 



About forty tertiary species have been described. 



P. PATULA, Linn. PI. 43, figs. 19-22. 



Brown, the ribs and nodules darker ; the much excavated 

 columella light chocolate-color, with a darker margin or semi- 

 lunate portion posteriorly; grooves within the outer lip dark 

 chocolate. Adults attain 4 inches in length. 



(lulf of California to Panama ; Isle of Bourbon (Deshayes) ; 

 Philippines (Cuming) ; West Indies. 



T lie re can be no doubt of the very extensive distribution of 

 this species in tropical and subtropical latitudes. Dr. Gould 

 has distinguished the Pacific specimens by characters derived 

 from depauperate or immature shells, under the name of P. pansa ; 

 but well-developed forms do not differ from those of the West 

 Indies. In young individuals the revolving ribs and tubercles 

 are much sharper than in adults. This species does not inhabit 

 the Mediterranean, although so stated by several authors. 



