298 MAZATlASr tru I VALVES 



third, in fresh condition, is immatnre, with the apex but slight- 

 ly recurved, and the radiating striae very coarse, while the 

 concentric laminae are but slightly developed. Epidermis in 

 shaggj^ layers between the laminae. Long. "34, lat. '3, alt. "18. 



Hah. — W. Indies, passim. — Cuba, Sagra. — Barbadoes, Dr. 

 Cutting. — Senegal, Adanson. — Loander, Tarns. — St. Vincent, 

 "W.Africa, Schmidt. — Lobos Is., Peru; on stones, among 

 coarse sand, 17 fm. ; Cuming. — Panama ; 14 sp. on stones 

 and shells near low water mark, with thin attached valves ; 

 C. B. Adams. — jMazatlan ; extremely rare, off Spondylus ; 

 L'pool Col. 



Tablet 1469 contains the 3 specimens. 



348, HiPPONTX PLANATUS, n. S. 



S. t. valde planatd, suhovali, alba ; vertice anfr. ii. rapide 

 augentibus, lateraliter incumbentihus, interdum persistente ; 

 t.juniore sparsim radiatim costatd ; postea laminis concentrieis 

 a sulcis radlantibus indsntatis, ad marginem t. junior e prof unde, 

 t. adultd vix serratis ; epidermide tenui, laminas intercalante ; 

 intus valde politd, cicatr. muse, angustd, circulari, regions 

 capitis interruptd ; margine producto, simplici, intus rotundato, 

 extus tenuissime serrato ; valvd inferiore tenui (t. juniorej 

 multo minore, usque ad regionem cicatricis solum continud. 



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. AU agree in the extreme flatness, torque-shaped 

 muscular scar, finel^^ cancellated laminae 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 fi'om even the flattest 

 specimens of H. autiquatus 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 long. '73, 

 lat. 64, alt. 17. 



JTab. — Mazatlan ; on Spondylus, extremely rare ; L'pool Col. — 

 Bay of Panama, T. Bridges, Mus. Cuming. 



Tablet 1470 contains the youngest and an adolescent sp. 



