Vol J, Grabau — Ordovician Fossils from North China (i) 87 



Sections of this species made so as to give the appearance of a centran siphuncle 

 (Plate VIII, fig. 5a) might be taken for A. richthofeni. They can however be readily 

 distinguished by the slight depth of the camene. In a transverse section of course, the 

 subcircular outline and submarginal position of the siphuncle readily distinguish this 

 species. 



Horizon and Localities: This species has been obtained from the Machiakou 

 limestone of the Kaiping basin in eastern Chihli province. It has been found at 

 Tangshan and more doubtfully at Machiakou (Survey expedition). A natural section, 

 apparently of this species, has also been obtained from the same horizon at Wen-Nan, 

 Mon-Yin-Hsien, Shantung (V. K. Ting coll.). 



Actinoceras nanum Grabau (sp. nov.) 

 Plate Yll, fig. 8; Plate ]X, tig. 2. 



• Shell slender, the longest specimen observed being about 70 mm. in length and 

 10 mm. in diameter at the basal end. It tapers at the rate of about 1 in 4. A second 

 specimen (Plate IX, fig. 2) about 45 mm. long, has a basal diameter of 6.5 mm. and 

 tapers at the rate of 1 in 5.75. A third specimen (Plate VII, fig. 8), the most perfectly 

 preserved, has a length of about 18 mm., its basal diameter is 6 mm. and its rate of 

 tapering 1 in 4. 



Siphuncle centran, small, the greatest diameter of the nummuli being 2.1 mm. 

 where that of the shell is 9.4 mm. ; strongly nummuloidal, contracting at the septa to a 

 diameter of 0.7 mm. Endosiphuncle subcylindrical, about 0.6 mm. in diameter. 



Septa gently concave. In the best preserved specimen they are 1.3 mm. apart 

 where the diameter of the shell is 10 mm. and practically the same where the shell 

 diameter is 8.5 mm. In another specimen, (Plate IX, fig. 2) the septal interval is 

 1.6 mm. where the shell diameter is 10 mm. and that of the nummuli about 2 mm. 



Stereoplasm shghtly developed or almost absent. When present, as in fig. 8 

 Plate VII, it is thickest near the siphuncle, but thins away rapidly before reaching this. 



This species is readily recognized by its tapering to a very narrow end which is 6 

 mm. or less, a diameter found in no other species in these rocks; by the very approxi- 

 mate septa (from 1.3 to 1.6 mm. where the shell is 10 mm. in diameter); and by the 

 minute centran siphuncle, which is smaller than that of any other species known from 

 these rocks. 



