512 Systematic Paleontology 



rounded ; hinge teeth apparently large, cup very distinct, pallial line very 

 distinct. 



" Dimensions. — Length about .6 in., width about .8 in. 



" Locality. — Dark marl, Crosswicks, N. J. My collection. 



" This is one of the finest casts I have ever seen from New Jersey. I 

 take pleasure in dedicating it to my friend, Dr. J. H. Slack, to whom I 

 am indebted for this and several other new species.''' — Gabb, 1860. 



Gabb's type is now in the collection of the Philadelphia Academy of 

 Natural Sciences. 



Shell thick, heavy, nacreous, transversely ovate in outline; umbones 

 prominent, inflated, inclined to be flattened upon their summits, feebly 

 opisthogyrate, placed backward two-thirds to three-fourths of the total 

 length; lunule and escutcheon ill-defined; posterior dorsal slope steeper 

 than the anterior; anterior end broadly rounded or obscurely truncate; 

 posterior end more sharply rounded; ventral margin approximately 

 horizontal or gently arcuate in the medial portion ; external surface finely 

 striated radially and concentrically sculptured by the incrementals ; par- 

 ticularly toward the ventral margin ; hinge taxodont, the teeth, when per- 

 fact, arcuate, and very prominent, raised a couple of millimeters above the 

 hinge plate, set in an anterior and a posterior series divergent at an angle 

 of 120° or 125° beneath the umbones ; anterior series numbering twenty to 

 twenty-five, becoming increasingly fine toward the beaks; posterior series 

 numbering ten or twelve, slightly less prominent toward the beaks; chon- 

 drophore narrow, spoon-shaped, directed forward from beneath the 

 umbones; muscle scars deep, roughly semi-elliptical, placed at the distal 

 ends of the hinge plate; pallial line simple, distinct; inner ventral margin 

 finely crenate ; immature forms relatively higher and less convex than the 

 adults. 



Nucula slackiana Gabb is abundant and readily recognizable. The shell 

 substance is very thick and frequently weathers in such a way as to give 

 an excellent cross section of the three layers which make up the shell — the 

 outer radially sculptured with the component prisms normal to the surface, 

 the inner, very thin, with the prisms laid parallel to the inner surface, the 

 middle layer which makes up the bulk of the shell with the prisms laid 



