268 Introduction to Animal Morphology. 



teeth on the free valve alone, allowing the valves to move 

 parallel to each other; the mantle lobes uniforal, dimyary. 

 These form one family (Hippuritidae) and eight genera, 

 from the cretaceous rocks, except the miocene Tamiosoma. 

 Sub-class 2. Exocardines shell not reticulated ; umbones 

 smaller, marginal; ligament external; valve-opening hinge- 

 like ; teeth usually on both valves. This includes the follow- 

 ing orders : 



Order i. Monomya (Lamarck) with one adductor, liga- 

 ment nearly internal ; mantle uniforal ; hinge nearly or quite 

 toothless, with one circumpallial nerve ; no pallial sinus nor 

 siphon, and an inequivalve shell. This includes the follow- 

 ing families : Anomiadae, with unsymmetrical body, no 

 labial palp, nor eyes ; arched hinge line, obsolete ligament, 

 and tin- under valve pierced for the calcified byssus ; rarely 

 '.vith transparent shells (Placuna). Ostreida?, with labial 

 palps separate from the gills ; foot obsolete ; ligament inter- 

 nal in one conical groove ; shells sessile ; lamellae rarely free 

 <-a), smooth (Grypha.-a). Spondylidae, with straight hinge 

 line ; ligament half-external ; a triangular hinge area on the 

 shell; teeth two, locking, rarely none (Pedum) ; foot small, 

 pointed. Radiilida?, with gaping shells ; long pallial tentacles ; 

 no ocelli ; foot not byssiferous (with a byssus, Stolicza); hinge 

 toothless. Vulsellidce, with long compressed shells ; narrow 

 gills, much produced posteriorly, united together and to the 

 mantle at their border ; rectum simple. Pectinidrc, with no 

 liin.^e area ; mantle with ocelli ; gills equal ; foot small, cy- 

 lindrical, sometimes byssiferous. The shells are auricled, 

 usually regular, smooth (Amussium), or radially ridged (Pec- 

 ten), rarely unsymmetrical (Hinnites). Aviculidae, with ter- 

 minal, not medial umbo, except in Crenatula ; shells one or 

 two-winged (Avicula), sometimes hammer-shaped (Malleus) ; 

 foot small, byssiferous, for which one valve is grooved. 



Order 2. Heteromya muscular scars double, unequal, 

 sometimes, as in Miilleria (fam. yEtheriidae), becoming mo- 

 nomyary in the adult. The shells of the rest are usually equi- 

 valve, or nearly so, the ligament external ; there are never 

 lateral teeth, and the cardinals are weak; mantle uniforal 

 (Pinnidae) or biforal, with a wide pedal slit (Mytilidae). This 



