EXPLANATION OF PLATE XVII. 



1, 2, 3. Round or Hard Clam, Venus Mercenaries, showing the different 

 parts of a bivalve shell. 



U. Umbones or bosses. The swelling part of bivalve shells near the beaks. 

 The highest points of the beaks are the summits. 



L. Lunule, a crescent-like mark or spot near the anterior or posterior slopes 

 in bivalve shells, sometimes called areola. 



D. D. Dorsal or superior border, near the bosses or beaks. 



V, V. Ventral or inferior border, or border lip, at the base of the shell, oppo- 

 site the beaks. 



A. Anterior or oral extremity, the part in which the ligament is not placed. 

 In a univalve it is the greatest distance from the apex. 



P. Posterior or anal extremity, that side of the bosses containing the ligament. 



Le. Length in bivalves is taken horizontally, or from the posterior to the an 

 terior margin ; in univalves it is takea perpendicularly or from the 

 apex to the base. 



Disk, the middle part of the valves. 



H. Heighth. T. Thickness, through the shell from disk to disk. 



Lig. Ligament, an external substance, uniting the two valves, and which in 

 fact is the true hinge ; the internal or cartilaginous part is often contin- 

 ued between the teeth. 

 The hinge is composed of the ligament, the cartilage and the teeth. 



C. Cardinal teeth, i. e. the serratures or dentations beneath the bosses. 



Lat. Lateral teeth, at the sides of the cardinal teeth. 



A. imp. Anterior muscular impression. P. imp. Posterior muscular im- 

 pression ; these indented marks upon the shell show where the adductor 

 muscles are attached. 



Pal. imp. The Pallial or marginal impression formed by the mantle of the 



animal. 



4. A Multivalve Shell, one composed of many pieces, as the Chiton. 



5. Fusus. A spindle-shaped UNIVALVE SHELL, showing the different parts. 

 Ap. Apex, or posterior part of a univalve shell, the point or nucleus of a 



shell, the top of Limpets and all univalves, and the bosses or beaks of 



bivalves. 



Sp. The Spire includes all the volutions except the body whorl. 

 S. W. Spiral whorls ; each complete turn is termed a whorl or volution. 



B. W. Body or basal whorl, is the last and usually much the largest. 



S. Suture, the line where the whorls of spiral shells meet or fit into each 



other. When grooved or furrowed it is said to be canaliculated. 

 Col. Columella or Pillar, the internal support round which the whorls wind. 



C. Lip. Columella, inner or pillar lip, folds over the lower part of the columella. 

 0. Lip. The outer lip is the external edge or termination of the last whorl. 



A. or M. Aperture, mouth or front, from which the body can protrude. 

 Ca. Canal, groove, or furrow in the beak as in Fusus, Murex, &c. ; in the 



Buccinum, Harpa, &c., it is only a notch, as in fig. 8. 



B. Beak, or rostrum, the continuation of the body whorl. 

 B. or A. The base or anterior part. 



6. A Turbinated Shell, (Paludina vivipara,) with the young shells. 



7. The OPKRCULUM, (door or cover,) closing the mouth, found in nearly all 

 predaceous univalves, and always attached to the foot of the live animal. 



8. Tiara, showing a turreted shell, with plaits or folds on the pillar; S, striee ; 



N. notch at the base, R. ribs, and T. tubercles. 

 9. Physa, showing the reverse or sinistral aperture. 

 10. Snail, (Helix anastoma depressum^ showing the reflexed lip and the teeth. 



