MAZATIAN BIVALVES 151 



veloped (as a sinking in tlie otlierrvise Avicula-shaped ligament) 

 at a very early stage ; afterwards a second, and so onwards. 

 The prismatic structure is not apparent at first, but soon 

 develops at the ventral portion in cells intermediate in size 

 between those of Avicula sterna and M. Mazatlanica, making 

 the transparent young valves very beautiful objects. The size 

 of the sheU when the ligament ints appear, varies greatly ; and 

 the rapidity with which one individual will completely alter 

 its form is most extraordinary. The largest valve measures "03 

 across ; an enormous specimen, long. 2'1, lat. V, alt. "5. 



One of normal shape „ „ 1*, „ "7, „ "24. 



A produced one . „ „ 1"53, „ "SI, „ 'IG. 



J«6.— Cuba, Sagra.—^t. Vmcent's, W. I.. W. B. Carpenter.— 

 Conchagua, Sinds. — Panama, Lieut. Wood. — Mazatlan ; not 

 uncommon in crevices of large shells, affixed by a short 

 strong byssus ; L'loool Col. — (Perna a.) Panama ; common 

 under stones and in crevices of rocks at low water mark ; 

 C. B. Adams. — La Paz ; Lieut. Green. 



Tablet 683 contains 7 pairs of valves, extremely young. — 

 684, 4 pairs and 2 pairs of valves, a stage older.— 685, 6 pairs 

 and 2 closed hinges, young.— 686, 3 pairs adolescent. — 687, 4 

 pairs adidt, greatly varying in shape.— 688, 2 sp. in situ, in 

 crevices of PatcUa Mexicana, and in dead Balanus ofi" do. 



206. IsoGNOMON Janus, n. s. 



I. t. rhomhoided, tenui, planatd, pallida ; valvd inferiori 

 IcBvi, superiori costis gracillimis, interdum ohsoletis, testd juni- 

 ore spini.f semituhiilis imhricatd ; haud aurita, angulo ad 

 umhones acuto ; marginihus ant. et post, suhparallelis, suhrectis : 

 cardine ligamento lot in Aviculd longo,fossihus 2ierpaiicis, mini- 

 mis, irregiilariter dispositis ; imp. muse, haud magno. 



SheU known from all the forms of I. Chemnitzii by its regu- 

 lar lozenge shape without ears, thin texture without concentric 

 ridges, light colour, and by the sculpture of the valves of 

 which the under is smooth, the upper ornamented with fine 

 radiating stria?, which in the young shell are crowded with 

 semitubular imbricated spines. The hinge line is narrow, 

 without pits when young, when adidt with a very few (4) small 

 ones at unequal distances. Muscular impression much smaller 

 than in I. Chemnitzii. The youngest shells are very ina;qui- 

 valve, and display capped umbos, consisting of Venus-shaped 



