ORDER I PRIONODESMACEA 385 
ous epidermis ; area amphidetic or obscure ; ligament parivincular, usually opisthodetie 
and external; gills usually filibranchiate ; mantle lobes without ocelli, more or less free, 
generally with the anal siphon complete and the branchial incomplete ; foot small, digiti- 
form, grooved, byssiferous ; monoecious ; mostly marine. 
Family 38. Modiolopsidae. Fischer (emend.) 
Shell Modioliform, usually equivalve, free, thin, with sub-equal adductor scars ; 
ligament deep-seated ; hinge edentulous or Dysodont ; sometimes byssiferous. Silurian to 
Cretaceous. 
The heavier forms show an obtuse ridge or two extending from the beaks toward the basal 
margin. The pedal scars are separate from and behind the anterior adductors. The forms 
included here appear to be 
the prototypes of the My- 
tilidae, from which they 
differ chiefly in those char- 
acters that are common to 
most of the ancient types, 
such as the sub-equality of 
the adductor scars and their 
more dorsal situation. The 
recent Jdas is very similar. 
Modiolopsis, Hall (Fig. 
677). Valves _elongate- 
oval, closed, with nearly 
terminal beaks, narrow 
hinge plate, and edentul- 
ous hinge. ? Ordovician ; 
Silurian. 
Modiomorpha, Hall. 
Similar, but with a wider 
hinge plate, and single, 

oblique, elongate, posterior Fic. 677. Fic. 678. 
ridge-like tooth. De-  Modiolopsis modiolaris, Myoconcha striatula, Goldf. Lower 
vonian Conr. sp. Ordovician ; Cin- Oolite ; Bayeux, Calvados. 1/j. 
: y cinnati, Ohio. 1/}. 
Myoconcha, Sow. (Fig. 
678). Hinge usually with an elongate cardinal, and a long, weak, lateral tooth in the 
right valve; otherwise resembling Modiolopsis. Carboniferous 
to Cretaceous. 
? Hippopodium, Sow. Thick, inflated, ovate, concentric- 
ally waved. Hinge with a long, blunt, oblique, cardinal 
tooth, or edentulous ; adductor scars strong. Jura. 
Modiolodon, Whiteavesia, Eurymya, Aristella, and Prolo- 
bella, Ulrich ; Goniophora, Phillips. Silurian and Devonian. 
Family 39. Mytilidae. Fleming. 
Shell equivalve, very inequilateral, heteromyarian, slightly 
yakig, typically Dysodont ; area amphidetic or none ; ligament 
et external, deep-seated; rarely with an alivincular 
internal resilium; pallial line simple; mantle lobes wnited 
; é below the anal siphon, otherwise free; generally byssiferous. 
Mytilus sublaevis, Sow. Great . 
Oolite ; Minchinhampton, Eng- Devonian to Recent. 
anh Ge Mytilus, Linn. (Fig. 679). Shell elongated, thin, with 
terminal pointed beaks; valves wider and rounded behind, gaping a little for the 
VOUT 2c 

Fic. 679. 
