298 MAZATLAN UNIVALVES 
third, in fresh condition, is immature, with the apex but slight- 
ly recurved, and the radiating strie very coarse, while the 
concentric laminee are but. slightly developed. Epidermis in 
shagey layers between the lamine. Long. 34, lat. °3, alt. °18. 
Hab.—W. Indies, passim. — Cuba, Sagra.— Barbadoes, Dr. 
Cutting.—Senegal, Adanson.—Loander, Tams.—St. Vincent, 
W. Africa, Schmidt— Lobos Is., Peru; on stones, among 
eoarse sand, 17 fm.; Cuming.—Panama; 14 sp. on stones 
and shells near low water mark, with thin attached valves ; 
C. B. Adams.—Mazatlan; extremely rare, off Spondylus ; 
LT’ pool Col. 
Tablet 1469 contains the 3 specimens. 
348, HipPpoNnyx PLANATUS, 2. Ss. 
H. t. valde planati, subovali, alba; vertice anfr.u. rapide 
augentibus, lateraliter incumbentibus, interdum persistente ; 
t. juniore sparsim radiatim costatd ; postea laminis concentricis 
a sulcis radiantibus indentatis, ad marginem t. juniore profunde, 
t. adulté vie serratis ; epidermide tenut, laminas intercalante ; 
intus valde politd, cicatr. musc. angustd, circulari, regione 
capitis interrupta ; margine producto, simplici, intus rotundato, 
extus tenuissime serrato ; valvd inferiore tenui (t. guniore) 
multo minore, usque ad regionem cicatricis solum continua. 
Three very small specimens of this species were found in 
the Spondylus washings; but Mr. Darbishire was fortunate 
enough to find a beautiful young shell in a Lithodomus bur- 
row in situ; too late for the list in the Proc. Zool. Soc. 
Afterwards some large adult upper valves were brought by 
Mr. Bridges. Allagree in the extreme flatness, torque-shaped 
muscular scar, finely cancellated laminze and rounded but 
spreading margin. The spiral apex, being recumbent and 
rather solid, has remained on two of the specimens, a very 
unusual thing in the genus. It is known from even the flattest 
specimens of H. antiquatus by the shape of the scar, spreading 
margin, recumbent apex, and sparse radiation of the earlier 
portion. The youngest shell is *07 across, with the Helicoid 
whirls about ‘015 in diameter. The most perfect specimen in 
Mr. Cuming’s collection (not the largest) measures Jong. °73, 
lat. -64, alt. 17. 
Hab.—Mazatlan ; on Spondylus, extremely rare ; L’pool Col.— 
Bay of Panama, 7. Bridges, Mus. Cuming. 
Tablet 1470 contains the youngest and an adolescent sp. 
