NOMENCLATURE. 5 



The radius* (adopting the name used by Bruguiere, 

 Lamarck, and others) differs remarkably in appearance 

 (though not in essence) from the walls or parietal portion, 

 owing to the direction of the lines of growth and the state 

 of its usually depressed surface. In the upper part the 

 radii overlie the alae of the adjoining compartments : in 

 outline (r, fig. 1, 2, 3), they are wedge-formed, with their 

 points downwards ; their summits (and this is often a useful 

 specific character) are either parallel to the basis or as in 

 fig. 1 and 2, oblique. The radii are sometimes not developed. 



The alee (so called by Dr. Gray) are overlapped by the 

 radii and by part of the walls ; they usually extend only 

 about half way down the compartment (a fig. 3, 4, 1) ; 

 their summits are either parallel to the basis or oblique. 

 The ala3 of the several compartments, together with the 

 internal, upper, thickened surfaces of the walls, against a 

 shoulder of which the sutural edges of the alee abut, have 

 been called (by Dr. Gray) the sheath {vagina). The upper 

 and greater portion of the sheath is marked by transverse 

 lines, caused by the exuviation of the opercular membrane, 

 as that membrane may be called, which unites the operculum 

 all round to the sheath, or upper internal surface of the 

 shell. 



The carina has always two alee, as in fig. 4. 



The carino-lateral and lateral compartments, have always 

 an ala on one (the rostral) side, and a radius on the other 

 (the carinal) side, as in fig. 3. 



The rostro-lateral compartment (when present) has always 

 radii on both sides, as in fig. 2. 



The rostrum has normally alee on both sides, as in fig. 4, 

 but very often from fusion with the rostro-lateral compart- 

 ments on both sides, it has radii on both sides, as in fig. 2. 



* The radii have been called byltanzani and De Blainville "arese depressse" 

 (the parietal portions of the compartments being the " arese prominentes") ; by 

 Poli, "areae interject* ;" by Gray, "sutures;" by Coldstream, "compartments 

 of the second order," (the parietal portions being those of the first order) ; by 

 some authors as " intersticia". I may here add that the scuta are the " ventral 

 valves" of Gray, the " anterior" of Ranzani, and the " inferior opercular" of 

 De Blainville : the terga are the " posterior valves" of Gray and Ranzani, but 

 the "superior opercular" of De Blainville: the rostrum, on the other hand,is the 

 "anterior valve" of Ferussac and the "ventral" of De Blainville; the carina 

 being the " dorsal valve" of the latter author. 



