486 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



cave; the peristomice has the form of a trapezoid; the peristome is absent. The 

 ovicell is small, entirely imbedded in the distal zooecium, not salient; its frontal 

 is perforated with numerous small pores. The median avicularium is rather large, 

 smooth: it makes a prominence in the peristomie in the form of a denticnlate/1 

 tube with its distal part. 



. fyw>=0.13mni. . f 2 =0.65-0.70 mm. 



Measurements. renstomice 7 /ooecia 7 



[tpe=0.l5 mm. us=0.3,j mm. 



Affinities. Under the microscope it is rather easy to instinctively separate 

 this species from Porella jacksoiiii-n. but upon analysis it is difficult to find the 

 clearly separating characters. It appears to differ from it in the small denticles 

 which often terminate the median avicularium and in the size and rarity of the 

 tremopores. 



The tangential section (fig. 5) is that of a tremocyst resting on an olbcyst; 

 the ovicell is here confounded naturally with the peristomie. 



The tremopores are tubular and as long as the thickness of the zooecium. 



Occurrence. Claibornian (Gosport sand) : One mile southwest of Rockville, 

 Alabama. 



Lower Jacksonian (Moodys marl): Jackson, Mississippi (common). 



Cotypes. Cat. No. 64146,' U.S.X.M. 



PORELLA JACKSONICA, new species. 



Plate (33, figs. 6-17. 



Description. The zoarium is bilamellar, formed of two lamellae, back to back, 

 difficultly separable; the fronds are flat or undulated. The zooecia are distinct. 

 very elongated, fusiform; the frontal is convex and terminated distally by an 

 enormous, oblique avicularian mucro making a projection at 45 and covering 

 in part the apertura ; the frontal is formed by a tremocyst with large tubular 

 pores resting on a thin perforated olocyst. The apertura is transverse, semilunar, 

 oblique, buried at the base of the peristomie. The peristome is not salient. The 

 ovicell is entirely imbedded in the walls of the distal zooecia and is not salient: its 

 frontal area is pierced with pores and very fragile. The median avicularium is 

 smooth, enormous, very salient: it opens on the <ide of the apertura. There are 

 some small, inconstant adventitious avicularia formed by the coalescence of two 

 tremopores. 



. ! hpe=Q.W mm. f/,3=0.5J-O.SOmm 



Mcusuri'mt'iitx. I enstomice ' Zooecium {-. 



^e=0.10-0.12 mm. 1^=0.^4-0.30 mm. 



Variations. This species is of a disconcerting irregularity and it is with much 

 trouble that we have been able to assemble the various forms which appear at first 

 sight absolutely distinct. The irregularity arises chiefly from the alteration dur- 

 ing fossilization, as is the habit in the species with great relief; the breaking of 

 :he median avicularium, the obliteration of the tremopores, and the rupture of the 

 avicularian area are other causes which give to the specimens absolutely divergent 

 aspects. Their illumination even under the microscope changes their aspect; the 



