298 CRETACEOUS PELECYPODA 



ciliated, those of the anal opening are slightly extensible, and finely ciliated; 

 labial palps two on each side, long (about 1-, or slightly less than the body), veiy 

 narrow, and linear in front and united above the mantle, broader and sul)-triau- 

 gular posteriorly, attached along the dorsal line only, but free Avith the hinder 

 portions ; one pair of gills* on each side, the outer smaller than the inner, both 

 entirely free, only attached along the upper or dorsal line, and in continuation 

 of this attachment lies the commissure -which separates the exhalant opening 

 I'rom the inhalant region ; the length of the gills is about equal to f th the 

 length of the body, or it is somewhat less. Each gill consists of a large 

 number of fillets, which are of two kinds, longitudinal and transverse. On the 

 upper portion of the gills the first are stronger, being at moderate distances, inter- 

 sected by vertical grooves ; towards the edges of the gills the fine transverse striation 

 becomes the more distinct one. Retractile muscles strong ; a thin muscle origi- 

 nates at the anterior end of the gills, continues anteriorly, becoming at the same 

 time thicker, and terminates at the anterior part of the palps ; it is with the lower 

 edge partially united to the anterior muscular impression, and it is the muscle which 

 produces the svipplemeutary small scar generally situated behind the large anterior 

 one, and is occasionally more or less confluent with it. This supplementary muscle 

 appears to have the object of producing a harmonious movement in the gills with 

 the palps. The visceral cavity did not oflFer any peculiarity ; the rectum lies, as 

 usually, along the xipper posterior edge and terminates in the exhalant opening. 

 The posterior lower portion of the body was in two specimens entirely filled with 

 undeveloped eggs. 



This description of the animal seems with very slight alterations to apply 



to all the typical species of unionin^ of H. and A. Adams, but in the American 



genus 3Iyceto2ms the authors note some very marked differences. It would appear 



from their description that there is no separate anal opening, as is, for instance, 



the case in Crassatella ; the outer branchiae are said to be entirely grown together. 



The foot is very long and cylindrical with a thickened knob at the end. It is 



evidently intended for boring in mud, similar to the same organ in the Solenid^, 



which use it by extending the thickened end into a point, then twist it round 



with a forward pressure, thus making a hollow as if executed with a borei', then 



expanding the end of the foot with force, so as to give it proper hold in the 



hollow made, after which the foot itself is contracted, and the whole animal 



advances in the direction where the hollow was made. I have little doubt that 



the Mycetopi and their allies use their club-shaped foot in a similar manner. The 



Uniones use their foot more for digging or making superficial furrows, forcing the 



removal of the mud by lateral pressure. Considering the above noted differences 



which are unquestionably very important, H. and A. Adams propose for that genus 



a special sub-family, mycetopin^. 



* In most of the specimens of U. miirginalis I found (in June) on the gills a pair of a veiy remarkable parasitic 

 Arachnoid, bclonginsj to the order ColojmJa, resembling in form Ixodes, but possessing only two distant occelli in 

 front. I shall take an early opportunity of describing these apparently very interesting animals. 



