202 SYNOPTIC COLLECTION. 



may bore into the same piece of wood, but these tunnels 

 do not, as a rule, come in contact. 



The second group of Pelecypods, or those possessing 

 transverse teeth on the hinge area, may have descended 

 from forms like Nucula or even from some simpler spe- 

 cies. 1 Nucula occurs in the ancient formations (No. 381, 

 JV. ventricosa Hall) and has continued slightly modified 

 to the present day (No. 382, N. tennis ; No. 383, N. mar- 

 garitacea Lam.). It is a smooth, symmetrical shell with 

 equal valves. Contrary to the usual rule the umbos are 

 directed towards the posterior end of the body which is 

 short and rounded, while the forward end is longer and 

 more pointed. The primitive hinge area is curved and 

 bears a few teeth which are at right angles to the antero- 

 posterior axis of the body. This primitive hinge area or 

 cardo is better seen in the larger shell, Area occidentalis 

 Phil. (No. 384). Here it is long and straight, and the 

 many transverse teeth are well developed. In this genus 

 the umbos are widely separated and the ligament lies 

 between them. In Nucula the hinge area has a triangular 

 pit for the internal portion of the ligament, called by 

 Dall the "resilium," which aids the external ligament in 

 uniting and opening the valves. The whole shell is made 

 of a pearly or nacreous substance, and in its young and 

 adult stages no prismatic structure is ever developed. 

 The fleshy animal is primitive in structure like its shell. 

 The edges of the mantle are free, without tentacles, and 

 are not drawn out to form a tube or siphon. Two adduc- 

 tor muscles are present, one at either end of the body. 

 The gills are in two pairs in the form of simple, straight, 

 and separate filaments. The young Nucula is active and 

 throughout life it never becomes attached. The foot has 

 a cleft and can be flattened into a disc and used in 

 crawling. 



1 For a discussion of the subject see Verrill, Trans. Conn. Acad. 

 Arts and Sci., X, part i, 1899, p. 45. 



