OBSERVATIONS ON LIVING SOLENOMYA. 2J7 



Nucula to those of certain other mollusks is interesting in the 

 fact that these mollusks are in every case low and aberrant 

 forms. He says: "The most striking peculiarities in the develop- 

 ment are connected \vith the formation and disappearance of 

 the tests. Outside of the group, so far as I have been able to 

 learn, Dondersia is the only other mollusk whose embryos are 

 known to be provided with similar tests." 



Dondersia is now regarded as belonging to the Amphineura, 

 but for awhile was looked upon as related to the worms. Drew 

 remarks that the young embryos of Dentalium bear certain 

 resemblances to the embryos of Dondersia, Yoldia and Nucula. 

 Here again a resemblance is seen to an aberrant group of mollusks 

 w T hose affinities were for a long time obscure. Drew also says 

 that a somewhat similar resemblance is noticeable in the case of 

 the embryo of Patella. Again a resemblance is found to a group 

 whose characters are archaic. Dall, in speaking of the Doco- 

 glossa, says the various forms manifest what may be termed a 

 peculiar persistency of immaturity when compared with other 

 groups of gasteropods. Korschelt, Patten, Fisher, Lankaster, 

 Pelseneer and others testify to the primitive characters of 

 Patella. In my paper on "An Early Stage of Acmcea," I hava 

 collected a number of extracts from the above authors in regard 

 to these low characters. 



Pelseneer agrees with Stempell that the characteristic features 

 of Solenomya represent the oldest living group of the lamelli- 

 branchs. He calls attention to the absence of a protractor pedis, 

 extensive overgrowth of ventral mantle edges, peculiar develop- 

 ment of the branchi-anal siphon, the form of shell and absence 

 of interlocking teeth, with position of ligament, remarkable 

 overlapping of periosteum, prismatic structure of limy shell, 

 rudimentary form of mouth lips and mouth appendages, almost 

 without winding of intestines, excessive elongation of the ven- 

 tricle, the presence of pericardial glands, position, size and 

 structure of gills, separation of kidneys without cross com- 

 munication. 



Interesting features will doubtless be revealed when the 

 embryology of Solenomya shall have been made known, and 

 with the abundance of one species on our coast, S. velum, we hope 

 before long light may be thrown on the subject. 



