156 



NEW YORK STATE MUSEUM 



Dictyonema retiforme Gurley. Jour. Geol. 1896. 4:96,308 

 Diet yon em a retiforme Pocta. Syst. Sil. Boheme. 1894. 8:192, 193 

 Dictyonema retiforme Freeh. Lethaea Pal. 1897. 1: 575, fig. 145 

 Dictyonema retiforme Grabau. N. Y. State Mus. Bui. 45. 1901. p. 134, fig. 27 



Dr Gurley has furnished in his manuscript, from the types and other 



specimens, the following" redescription of this form, which is important as 



being the genotype of Dictyonema : 



Polypary rather strongly radiate, with the branches usually about 

 0.8 mm wide (a few as narrow as 0.6 mm, a few swelling out to 1 mm, par- 

 ticularly immediately below a bifurcation); about 15—17 in 25 mm of width, 

 in the basal portion, and in young specimens frequently somewhat more 



slender, more tortuous (with slight tend- 

 ency to zigzag) and somewhat farther 

 apart ; the interspaces consequently as 

 wide as, or slightly wider than, the 

 branches. Dissepiments mostly trans- 

 verse (some are slightly oblique, a few 

 very oblique) ; generally slender (about 

 0.2 mm), but a few reach 0.3 mm or 

 rarely 0.4 mm. Meshes mostly oblong ; 

 a number of careful measurements lias 

 shown me that the most usual (jhe typi- 

 cal) length is on the average 1.5 mm 

 (between 1 and 2 mm), but longer ones 

 are seen, from covering up or destruction 

 of the intervening dissepiments, which 

 condition in favorable cases can be 



Fig. 64 Dictyonema retiforme Hall. Enlarge- .... 



ment (x 5) of portion of specimen from the Rochester prOVeil. Rai'dV tWO SUCCeSSlYe dlSSeDl- 



shale at Lockport, N. Y. Note the crossing dissepiments 1 J I 



at the left ments are not farther apart than 0.5 mm. 



This almost invariably results from the two dissepiments diverging from a 

 common point of origin on one branch. 



Position and locality. In New York this form is found in the Roch- 

 ester shale, principally between Rochester and Niagara Falls. It occurs 

 also in the Lockport limestone at Hamilton, Ontario, ami Freeh cites it 

 also (probably erroneously) from Missouri. Its types are in the American 

 Museum of Natural History. 



It is by no means so common in our Niagara beds as one. should sus- 

 pect from its long bibliographic list or conclude from the direct statement 



