152 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 55 



figure 2 is typical, all show a lamellar structure with indications of 

 more or less numerous and scattered, very minute pores or tubules 

 passing without interruption through one lamella. In some sections 

 the spots indicating the tubules are arranged in rows radiating from 

 the beak of the shell to the margins, but no other regular arrange- 

 ment can be seen. The great mass of the shell is made up of a com- 

 pact, finely granular base with dark spots and occasional minute 

 crystals of calcite — a ground-mass which, under the microscope, 

 appears very much like that of a fine argillaceous shale. 



The Ordovician Protremata have a clearer, more crystalline aspect 

 or color than the Cambrian Billingsellidse — a difference which prob- 

 ably indicates either a purer lime composition for the former or 

 more probably a higher percentage of calcium phosphate for the 

 latter. In chemical aspect the shells of the Billingsellidge appear to 

 resemble those of the Atremata and Neotremata more closely than 

 do the Orthidse. Analyses of the respective shells would be neces- 

 sary to prove these relations, but to note them is interesting in view 

 of the possible derivation of the Billingsellidse from the Atremata. 



In the Cambrian articulate genera, with the possible exception of 

 Syntrophia and Huenella, there is an entire absence of the minute,, 

 fibrous structure so characteristic of most, if not all, orthoids. But 

 these two representatives of the Pentameracea greatly resemble each 

 other. Thus sections of the shell of Huenella abnormis (Walcott) 

 of the Upper Cambrian (see pi. 12, fig. 9) and Syntrophia lateralis 

 (Whitfield) of the Lower Ordovician (see pi. 12, fig. 7) show the 

 same radial arrangement of the pores seen in the Billingsellidse, 

 but the shell structure is fibrous and the rows are coincident in direc- 

 tion with the fibers. Upon closer study this apparent fibrous struc- 

 ture can be resolved into more or less parallel bands or walls of shell 

 substance separating rows of closely arranged, rectangular, pore-like 

 spaces. These spaces may be seen distinctly in thick sections, but 

 when the section is made sufficiently thin to give a clear image under 

 very high power, the pore structure disappears. 



Sections of the linguloid genera were also prepared and studied, 

 but the thinness of the shells and their phosphatic character prevented 

 very satisfactory results. The irregular large tubules mentioned by 

 Dr. Mickwitz [1896] are beautifully shown in the sections of Obolus 

 apollinis before me. Some of the tubules penetrate several lamellae 

 of the shell and suggest the tubules of some of the orthoids. (See 

 figures II and 12, pi. 12.) The same general structure, with the 

 exception of the larger tubules, appears to be characteristic of all of 



