294 MOPALIA. 



Genus XXIX. MOPALIA Gray, 1847. 



Mopalia GRAY, P. Z. 8. 1847, pp. 65, 69, 169, (exclusive of sec- 

 tion * *). Type Ch. hindsii. CARPENTER MS. and in Dall, Proe. 

 U. S. Nat. Mus., 1878, p. 3Q3.Molpalia GRAY, Guide Syst. Dist. 

 Moll. B. M., p. 184 (err.typ.), 1857. GOULU, Otia, p. 118. 

 Osteochiton DALL, Proc. U. S. Nat. Mus. 1886, p. 211. Type M. 

 sinuata Cpr. 



Valves normally proportioned, transverse, not beaked, exposed, 

 the girdle somewhat encroaching at the sutures. Sinus small ; inser- 

 tion-plate of anterior valve rather long, sharp, slit into nearly 

 smooth teeth which are somewhat thickened at the edges of the slits, 

 the latter being normally 8 in number and corresponding in position 

 to external ribs. Median valves having similar 1-slit insertion- 

 plates. Posterior valve depressed, the mucro post-median, insertion 

 plate rather sharp, smooth or roughened, having an oblique slit on 

 each side (rarely doubled) and a larger sinus in the middle behind. 

 Girdle wider at the sides than in front, leathery, more or less hairy, 

 the hairs simple ; with or without sutural pores. 



This gen us is known only from the shores of the northern Pacific, 

 extending from Lower California to Alaska and Japan. It may be 

 regarded as the most primitive of the existing genera of Mopaliidce; 

 Pladphorella, and Plaxiphora being special modifications of the 

 Mopalia type. The small additional slits occurring in some forms, 

 such as M. acuta, are cases of partial reversion to the multifissate 

 ancestral form which gave rise to the family. 



Two species of Mopalia were placed by Carpenter in Pladphorella 

 on account of their possessing sutural pores. Dall justly remarks 

 that they differ from the true Placiphorellas, and he proposes Osteo- 

 chiton as a section of that group for their reception. It is my belief 

 that these species have no near alliance with Pladphorella, but that 

 they are merely modified Mopalise. The gathering of a few hairs or 

 spicules into a pore at the sutures is no uncommon occurrence, and 

 in some cases (such as Ceratozona) it is not constant even as a speci- 

 fic character. In Mopalia this modification will probably be found 

 to occur in all the forms in which the girdle is nearly naked. In a 

 small form of M. muscosa from Bolinas Bay the pores are sometimes 

 present at all of the sutures, and sometimes at only two or three on 

 one side, entirely absent on the other side. Sometimes they bear 

 each a large hair, sometimes none. It is therefore obvious that we 

 can make no disposition of Osteochiton except to consider it aiL 

 absolute synonym of Mopalia. 



