WIGLEY and STINTON: REMAINS FROM MARINE SEDIMENTS 



waters, but also showed special sediment require- 

 ments, were: Bathyarca pectunculoides, Cuspi- 

 daria perrostrata, Limatnla suhauriculata, and 

 Thyasira gouldi. Conversely, those bivalves 

 that occurred in eurythermic habitats (and 

 coarse sediments) were: Ensis directus, Lyon- 

 sia hyalina, and Musculiis niger. 



Relations of pelecypod distributions with bot- 

 tom sediments are discussed below. 



Species-Sediment Relations 



Shells from 89 9^ of the 35 more common pele- 

 cypod species represented in the area were 

 found in several different sediment types; only 

 11% were associated exclusively with one sed- 

 iment type. All of the common species were 

 primarily in sediments in which sand or silt was 

 the chief constituent. Species frequently taken 

 in sand sediments were: Ensis directus, Limat- 

 ula suhauriculata, Lyonsia hyalina, and Muscu- 

 lus niger. Species most commonly found in silty 

 sand and sand were: Bathyarca pectuncjiloides, 

 Cuspidaria perrostrata, Cyclopecten thallasimis, 

 Limopsis sulcata, Nucula proxima, N. tenuis, 

 Nuciilana acuta. Pandora gouldiana, Phacoides 

 filosus, Pitar morrhuana, Spisula solidissima. 

 Thyasira plana, and Venericardia borealis. The 

 only two species found mainly in sandy silt or 

 silty sand were Limopsis sulcata and Thyasira 

 gouldi. The absence of Astarte undata in the 

 center of the area is probably due to the presence 

 of fine-grained sediments there. All the remain- 

 ing common species were collected from several 

 different sediment types. 



The presence of a narrow band of sand ex- 

 tending parallel to the depth contours near the 

 outer margin of the shelf (Figure 2) appears to 

 be a major feature affecting the distribution of 

 many species having a narrow-band distribution 

 (Figure 9). 



GASTROPODS 



Remains of gastropods formed an important 

 component of organic origin, but compared with 

 other mollusks they were far less common than 

 pelecypods, but considerably more abundant 

 than scaphopods and cephalopods. Gastropods 



were widely distributed throughout the area and 

 ranged in density (all species combined) from 

 to slightly over 1,000/m-. All remains were 

 shells, except for one operculum of Polinices 

 dupUcata. 



Forty-four species of gastropods were found 

 in the samples. Some typical examples are 

 shown in Figure 11. A large majority of spe- 

 cimens were small, less than 1 cm in shell height. 

 Some of the smallest specimens, averaging be- 

 tween 2 and 5 mm, were Alvania carinata, Cyl- 

 ichna alba, Retusa obtusa, and larval forms, pre- 

 sumably of Thais. The larger species, averaging 

 between 1 and 5 cm in greatest dimension, were: 

 Buccinum undatum, Colus pygmaeu^, Crucib- 

 ulum striatum, Crejridjila fornicata, and Nassar- 

 ius trivittatus. 



Gastropod occurrence records are listed in 

 Tables 8 and 9. Table 8 gives the species-station 

 record for the 24 more common species. Table 9 

 lists the occurrence record for species taken at 

 only one station. 



Distribution and Density 



Gastropod shells (all species combined) were 

 rather widely distributed throughout the area, 

 occurring at 80% of the stations. Highest con- 

 centrations (250 to 1,050/m-) were in the cen- 

 tral part of the area in a lens-shaped patch at 

 depths between 40 and 80 m (Figure 12). 



Although gastropod shells were collected at all 

 depths, the average concentration increased in 

 each 10-m depth class from 20 m down to about 

 80 m (Table 10). Concentrations were lower 

 in deeper water, except for a zone of greater 

 density between 175 and 250 m. Alvania cari- 

 nata and Cylichna goiddi made up the bulk of 

 the gastropod remains in the shallowwater zone: 

 Mitrella zonalis, a gastropod larva, and a variety 

 of species accounted for the high abundance in 

 the deepwater zone. 



Relations of Density to Sediments 



The density of gastropod shells as a group was 

 related to sediment type only in a rather general 

 way. No gastropod shells were found in the 

 coarse sand or gravel-sand substrates. Concen- 



17 



