MAZATLAN BIVALVES 151 



veloped (as a sinking in the otherwise 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 shell when the ligament pits appear, varies greatly ; and 

 the rapidity with w r hich one individual will completely alter 

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

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



One of normal shape 1', '7, "24. 



A produced one 1'53, '51,,, '16. 



Hob. Cuba, Sagra. St. Vincent's, W. I.. W. B. Carpenter. 

 Conchagua, Hinds. Panama, Lieut. Wood. Mazatlan ; not 

 uncommon in crevices of large shells, affixed by a short 

 strong byssus ; L'pool 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 adult, greatly varying in shape. 688, 2 sp. in situ, in 

 crevices of Patella Mexicana, and in dead Balanus off do. 



206. ISOGNOMON JANUS, n. s. 



I. t. rkomboided, tenui, planatd, pallidd ; valvd inferiori 

 IcBvi, superiori costis gracillimis, interdum obsoletis, testa juni- 

 ore spinis semitubulis imbricatd ; Jiaud auritd, angulo ad 

 umbones acuto ; marginibus ant. et post, subparallelis, subrectis : 

 car dine ligamento ut in Avicula longo,fossibus perpaucis, mini- 

 mis, irregulariter dispositis ; imp. muse, hand magno. 



Shell 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 strise, w^hich in the young shell are crowded with 

 semitubular imbricated spines. The hinge Ijne is narrow, 

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

 ones at unequal distances. Muscular impression much smaller 

 than in I. Chemnitzii. The youngest shells are very inseqai- 

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



