214 MAZATLAN UNIVALVES 



Tliis species scarcely differs from P. nigropunctata, Sow. 

 except in tlie entire absence of black dots round tlie margin. 

 Shell with veiy numerous fine ribs, with still finer ones between, 

 sometimes slightly nodidous. Outline more or less oval, more 

 or less conic ; growth regular. Hole large, subcentral, deeply 

 chiseled, oval, constricted in the middle externally, where it is 

 bilobed. Colour olive green, often stained with red when 

 polished. Siu'face frequently eroded. Interior from pale to 

 dark green ; margin crenated by the ribs ; callus rather thick, 

 radiately corrugated, not bounded by a dark line. jNIuscular 

 impression distinct. The young shells (which were very rare) 

 are shaped like the adult, with the hole even larger in propor- 

 tion. The largest sj). measures long. 2'2, lat. I'GG, alt. '8. 



Sah. — Panama ; in exposed situations at low water ; Cuming. — 

 Do. ; common, on a ledge of rather smooth rocks, between, 

 half tide and low water mai*k, in a place somewhat exposed 

 to the sea ; C. S. Adams. Mazatlan ; very common ; 

 L'pool Col. 



Tablet 1021 contains 10 sp., various ages, normal state, — 

 1025, 4 do., elongated foi-m. — 1026, 4 do. flattened form. — 

 1027, 3 do. rounded form.— 10-28, 2 do. conical.— 1029, 2 do. ribs 

 strong. — 1030, 2 do. ribs faint. 



Tablet 1031 contains 2 sp. surface cleaned, shewing red tint 

 beneath. — 1032, 5 sp. shewing shades of colour within. — 1033, 

 2 sp. diseased from worms, &c. — 1034, 1 sp. hole irregular 

 through accidental breakage. 



Tablet 1035 contains a remarkable monstrosity with 2 holes. 

 The first is in all respects normal. The second is adjacent 

 anteriorly, perfectly rouud, with a thick callus within, imited 

 to that of the normal hole posteriorly, anteriorly rather 

 pointed.* 



• A similar monstrosity appears in a specimen of F. omata in Mr. jS'uttall's 

 collection. The shell is abnormally conical, with the hole nearly round instead 

 of oblonp, bilobed. The second hole commences 3-5ths down towards the anterior 

 margin, is very minute, and linds its exit close to the other at the apex. It is 

 surrounded bv a thick callus. A still more remarkable monstrosity, without a 

 hole at all, exists in U'Orbigny's Col. v. JS. M. Cut. D'Orb. Moll. p. 51, no. 435. 



