La MELLIBRANCHIATA. | LOWER PALAZOZOIC MOLLUSCA. 265 
Position and Locality—Lower Ludlow mudstone of Green Quarry, Leintwardine; Lower Ludlow rock, 
Garden Quarry, Aymestry; schists above Park Lane, Llandeilo; Bala schists, Cyrn y Brain, Wrexham, Den- 
bighshire. 
2nd Ord. Iseproxoriia (M°Coy). 
Ref. and Syn.—Etym. ioos equal, and Zpa, with dos entire, and reid a scar (in allusion to the entire pallial 
scar and equal valves). 
Shell equivalve, symmetrical; natural position nearly vertical; animal haying the mantle usually open 
throughout, not forming siphons; the pallial impression always simple and entire; two or more muscular 
impressions in each valve. 
These shells resemble the Macrotrachia in being equivalve and symmetrical, and maintaining when alive 
a nearly vertical position, but differ from them and agree with the Plewroconcha in the mantle being open and 
seldom forming even rudimentary siphons, and the shell always shewing the pallial impression entire, while they 
differ from the Plewroconcha in being equivalye, symmetrical, and never attached by the substance of the valve, 
and but rarely by a byssus—thus naturally connecting the two extreme types, and, like most connecting groups, 
containing a greater variety of generic and specific types than the extreme groups which it connects. The 
greater number of known bivalves belong to the [sedrolotila. 
The order contains the following families: 1, Limidw; 2, Mytilide; 3, Arcide; 4, Nuculide; 5, 
Trigoniide ; 6, Unionide ; '7, Lucinide ; 8, Cyclaside; 9, Cyprinide; 10, Astartide; 11, Tridacnide ; 
12, Cardiide. 
2nd Family. MYTILIDA. 
Shell elongate, oval, narrow in front, widening posteriorly, more or less closed, (equivalve, pallial impression 
entire), cartilage very long, marginal or nearly so, supported by an internal plate nearly parallel with the hinge- 
line ; beaks close to the anterior end or terminal; two or three muscular impressions in each valve. Animal: 
mantle more or less open; gills fringed or entire; mouth with appendages ; foot narrow, tongue-shaped, with a 
fibrous byssus at its base to attach the shell to foreign bodies, having a small sinus in the anterior part of the 
ventral margin, from which an oblique impressed furrow extends towards the beak, defining the anterior lobe. 
Distinguished from the Aviculidw by being equivalve and having a strong anterior as well as posterior 
adductor muscle. 
Genera:—1, Mytilus; 2, Lithodomus; 3, Modiolopsis; 4, Dreisena; 5, Hippopodium; 6, Pinna ; 7, 
Dolabra ; 8, Myaphoria; 9, Anodontopsis; 10, Lyrodesma (Actinodonta) ; 11, Clidophorus ; 12, Tellinomya ; 
13, Orthonotus. 
Mytitus ? uNGuIcuLAtTusS (Salt.) 
Ref—Id. id. Salt. Mem. Geol. Surv. Vol. IT. pt. 1. t. 20. f. 6. 
A small specimen, too imperfect for description, in the decomposing schists of Plas Madoc. It does not 
seem to belong to the genus Mytilus. 
Genus. MODIOLOPSIS (fall) as here restricted. 
Syn. < Cypricardites Conrad. 
Gen. Char.—Shell equivalve, very inequilateral, transversely elongate, diagonally gibbous; beak small 
near the anterior end; anterior end small, rounded, often separated from the body of the shell by a shallow, 
oblique concavity, extending from the byssiferous sinus in the margin; posterior end very broad, obliquely 
subtruncate, or rounded ; hinge-line elevated, more or less compressed, nearly as long as the shell is wide ; the 
margin simple and erect ; shell very thin, only marked by minute lines of growth; one large, oval, muscular 
impression a little in front of the beak, and one large superficial posterior one. 
[Fasc. 11.] Mm 
