115 



thesized from what was found in the literature concerning closely-related 

 sub-tropical and tropical species. It must be realized, however, that it is 

 known that development can differ radically within the same genus of 

 mollusks. For this reason, 40 species of important lamellibranchs were 

 examined by the author and W. K. Okkleman as to prodissoconch type. 



V. Intermediate Shelf, 27 to 65 Meters: 



The assemblage characterizing this environment actually becomes im- 

 portant as a distinct entity at about a depth of 39 meters, especially in the 

 southern Gulf. In the northern Gulf, the change is more gradual. Although 

 10 stations were taken in depths of 27 to 38 meters, only 38 species were 

 found, none being confined to these depths. Nineteen of these species 

 were more abundant in the shallower waters, while 19 were more com- 

 monly taken in the deeper waters. This overlap in depth ranges plus the 

 lack of large numbers of individuals and species indicates that there is a 

 transition zone between the major assemblages of the nearshore and 

 intermediate shelf depths. This transition zone was more distinct in the 

 southern Gulf, as the sediments were much finer than those inshore or 

 offshore, ranging from silty sand to silty clay. Reduced wave action 

 permits the accumulation of finer sediments along with greater amounts 

 of organic matter within the sediments. The larger amounts of organic 

 matter can apparently support a large population of Penaeid shrimp, 

 since the 27 to 38 meter depth zone is the primary inshore shrimp grounds 

 for both the Gulfs of Mexico and California. The shrimp and the more 

 common invertebrates found at these 10 stations, the echinoid, Lovenia 

 cordiformis, the scaphopod, Cadulus perpusillus, and the pelecypod, 

 Nuculana elenensis, are all deposit feeders, well adapted to feeding in this 

 habitat. 



This same transition zone was also observed in the northern Gulf of 

 Mexico, wherever fine sediments occurred inshore of major sand bottom 

 areas resulting from lowered sea level (Parker, 1960; and Curray, I960). 

 As in the Gulf of California, the northern Gulf of Mexico intermediate 

 shelf assemblage was most distinct in depths of 39 to 65 meters on the 

 relict sand deposits. Only off Texas where deposition has been more rapid, 

 and fine sediments occur uninterrupted across the shelf, was the inter- 

 mediate shelf assemblage continuous from 20 to 65 meters. The assemblage 

 in this clayey zone in the Gulf of Mexico was also characterized by un- 

 common and widely-dispersed deposit feeders. The number of living 

 animals in 1/5 m- Van Veen samples from this transition environment off 

 Rockport, Texas averaged only 2.2 individuals per station or about 12 



