DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



105 



Fig. 4. Neilonella salicensis. A series of shells, from Atlantis II station 73 

 from the North Atlantic Basin, in lateral view from right side to show 

 change in shape with increasing size. Scale = 1mm. 



Shell description (Figs 2-A) 



Shell robust, moderately elongate, moderately wide, ornamented 



with concentric ridges sometimes in groups of three to four well- 

 defined ridges alternating with two to to three slighty less 

 well-defined ridges slightly wider spaced than the former, light to 

 dark straw-coloured periostracum; umbos moderately prominant, 

 somewhat anterior to mid-line (postumbonal length c.54% of total 

 length), inward and slightly anterior facing; dorsal shell margins 

 deep set close to umbo but no lunule or true escutcheon, antero- 

 dorsal margin slopes down to join anterior and ventral margins in 

 smooth curve, postero-dorsal margin almost straight, may be 

 slightly angled at posterior limit of hinge short of acutely rounded 

 posterior margin; posterior limit of shell somewhat dorsal to mid 

 horizontal line, anterior limit of shell at mid line; posterior margin 

 of smaller specimens less acutely rounded; hinge plates elongate, 

 moderately broad, increasing slightly in width distally, anterior 

 and posterior hinge teeth series meet below umbo with no edentu- 

 lous space between, teeth numerous, up to 19 in posterior series 

 and 14 in anterior series depending on size of specimen, obtuse 

 chevron-shape, proximal hinge teeth very small, those more distal 

 elongate and so closely articulated that it maybe impossible to 

 separate valves without damage to them; internal ligament 

 (resilium) microscopic, present ventral to umbo at outer margin of 

 hinge plate, external ligament opisthodetic, relatively short, short 

 anterior part present. Maximum total length of present specimens 

 8.0mm. 



As in most deep-sea protobranchs, there is a change in the shape 

 of the shell outline as growth occurs (Figs 4 & 5), although this is not 

 as marked as in many protobranch species (e.g. Ledella spp. Allen 

 and Hannah, 1989). The ratio of postero-umbonal length to total 

 length increases and at the same time the height to total length 

 decreases so that larger shells (>3mm total length) are more 

 posteriorly attenuate than smaller specimens. There is a fair degree 

 of variation in the ratios which is not related to the different 

 populations sampled. Unusually in some samples the length fre- 

 quency histograms are skewed to the right (Fig. 6) and with clearly 



ii o 70-J 

 TL S 



60 - 



PL , 

 TL " 



50 



Length (mm) 



Fig. 5. Neilonella salicensis. The ratios height to total length (H/TL)(large symbols) and post-umbonal length to total length (PL/TL)(small symbols) 

 plotted against total length to show changes in shell proportions with increasing length. Open circles are specimens from Biogas III station DS 49 West 

 European Basin, closed circles from Sarsia Station S44 West European Basin and closed squares, from Chain 58 station 103 North America Basin. 



