NUMMULITIDAE — AMPHISTEGINA; HETER08TEGINA. 57 



Test equally biconvex; periphery acute, with a narrow carina, somewhat 

 lobulate; umbilical region depressed, with a small central boss of clear shell 

 material, not extending out to the general contour of the test; retral processes 

 about one-fourth the width of the chamber, in depressed channels above the 

 sutures, the intermediate portions being raised, inflated, gradually increas- 

 ing in width toward the periphery; wall smooth, translucent, with very minute 

 punctjB; aperture a series of small rounded pores at the base of the apertural 

 face of the chamber. 



Diameter up to 0.50 mm. 



P. advena, while not as common in this region as the two preced- 

 ing species, has nevertheless been found at several stations. That it 

 is not the same as Munster's Rohidina subnodosa will be seen by a 

 comparison of our species with the figures of the type of Miinster. 

 While his figures are very small, the general contour of the test in 

 apertural view shows the great difference between these two species. 

 Munster's figure shows a test rhomboid in apertural view, with the 

 greatest width at the umbilical region, while in our species the 

 umbilical region is depressed. The specimens in the Tortugas region 

 are very constant in their characters. The specimens referred to Poly- 

 stomella subnodosa from the general Indo-Pacific region are probably 

 either P. advena or a closely allied species. 



Genus AMPHISTEGINA d'Orbigny, 1826. 

 Amphistegina lessonii d'Orbigny. 



Specimens of Amphistegina are abundant at many of the stations 

 in the Tortugas collection. D'Orbigny referred the West Indian 

 Amphistegina to his species Amphistegina gibbosa. Many of the 

 specimens from this region are thicker than is typical of those of 

 some parts of the East Indies. A careful study of abundant material 

 from the two general regions may show that there are distinctions 

 which are constant in their character. The West Indian specimens 

 are all comparatively small, none exceeding 2 mm. in diameter. 



Genus HETEROSTEGINA d'Orbigny, 1826. 



Heterostegina antillartim d'Orbigny. 



(Plate 10, Figure 5.) 



Heterostegina antillarum d 'Orbigny, in De la Sagra, Hist. Fis. Pol. Nat. Cuba, 1839, " Forami - 

 niferes," p. 121, pi. 7. figs. 24, 25. 



There are single specimens from two stations which are Hetero- 

 stegina and may be referred to d' Orbigny 's species, although they 

 are not so fully developed as shown in his type figure. Heterostegina 

 in the present oceans is limited to warm, shallow waters, such as are 

 characteristic of coral reefs. The development in the tropical Atlantic 

 is very small compared to that of the Indo-Pacific, although in the 

 Eocene of the West Indian region there is a fine development of the 

 genus. 



It is worthy of note here that these two specimens were from the 

 two stations where the temperature is abnormally high for the region. 



