41 



It has generally been assumed by geologists that in the 

 tectonic movements of the major continental masses wc have 

 an ample explanation for the principal changes in sea-level, 

 while the great ocean basins have remained relatively undis- 

 turbed. Recent studies as summarized by Menard (1955) 

 indicate that tectonic activities on a colossal scale have taken 

 place in the Pacific. Off Cape Mendocino on the coast of Cali- 

 fornia a great fault line has been discovered which extends 

 1,400 miles to the westward, flanked by submarine cliffs and 

 mountains rivalling those present on the continent. The ocean 

 floor to the south of the fault lies at a level several thousand 

 feet below that to the north. Since several such fracture zones 

 have been discovered it is clear that these disturbances must 

 have profoundly affected the oceanic level . Some of the sub- 

 marine mountains are flat on top, indicating they may have 

 at onetime been islands, possibly at a time when the extensive 

 submarine canyons fronting most of our larger rivers were 

 being excavated by rapid flowing streams. 



It is clear from the above that changes in sea-level, together 

 with the resulting terrace formations, are probably due to a 

 variety of interacting factors. 



GEOGRAPHIC DISTRIBUTION OF Thais lamellosa 



In the older references to the distribution of this species its 

 range is listed as extending as far southward as Santa Barbara, 

 but it now seems to be agreed that there is no record of its 

 presence on our coast southward of Santa Cruz in the vicinity 

 of Monterey. 



To the northward, populations of lamellosa are distributed 

 along the coasts of California, Oregon, Washington, British 

 Columbia and Alaska. In the latter area lamellosa appears to 

 be present throughout the Aleutian Islands, with an extension 

 into the Kurile Islands. All authorities appear to be in agree- 

 ment that this species ranges northward along the shore of 

 Bristol Bay to Port Clarence, but the writer has had no ma- 

 terial for comparison from this section of Alaska. 



