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 which one individual will completely alter 
its form is most extraordinary. The largest valve measures °03 
across ; an enormous specimen, Jong. 2°1, lat. 1°, alt. °5. 
One of normal shape * oy ML ee neg nae 
A produced one z vim DON sitet Ol aoe 
- Hab.—Cuba, Sagra.—St. Vincent’s, W. 1.. 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 roeks 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, im 
crevices of Patella Mexicana, and in dead Balanus off do. 
206. IsogNoMON JANUS, 7%. S. 
I. t. rhomboidea, tenui, planatd, pallida; valvd inferiort 
levi, superior costis gracillimis, interdum obsoletis, testa guni- 
ore spinis semitubulis imbricataé ; haud auritd, angulo ad 
umbones acuto ; marginibus ant. et post. subparallelis, subrectis : 
cardine ligamento ut in Aviculd longo, fossibus perpaucis, mini- 
mis, irregulariter dispositis ; imp. muse. haud 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 striew, which in the young shell are crowded with 
semitubular imbricated spines. The hinge line 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 inequi- 
valve, and display capped umbos, consisting of Venus-shaped 
