BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



liaaia 



Basis 



Structure of the individual compartments. If (ho basal margin of a compartment, 

 for instance of Inilunns tintinnabnlitm, be examined, it, appears sullieiently compli- 



cated, being composed of an outer and inner 

 lamina, separated by longitudinal septa, 

 which are denticulated at their bases; 

 and the tubes formed by these longitudinal 

 septa are crossed by transverse septa. On 

 the other hand, in some cases, as in the 

 genera ChlJnniuilns and Klminin.t, each com- 

 part ment consists of a simple shelly layer. 

 These two extreme states graduate into 

 each other tin different, genera V we have, 

 firstly, on the internal surface, unite irreg- 

 ular points and ridges, these become regular 

 i in some other genera \ causing the internal 

 surface to be longitudinally ribbed: then 

 these ribs themselves become finely fur- 

 rowed on their sides and at their lower cuds, 

 producing sharp, minute ridges, the ends 

 of which 1 have called the denticuli; and 

 lastly, some of the denliculi on the adjoin- 

 ing longitudinal septa become united into 

 a solid layer, forming the internal lamina 

 i -I the wall. (Darwin, p. -13.) 



FIG. l. W.M.I. OK A BALANOHOBPH UAKNACI.K. 

 a. a. AI..K. /i. />. P.VKIKTKS. r. r.RAWi. I'ART 

 OK Tiir. SHEATH is \ i^ir.i.i: IN THE DKIKICE. IN 



MAI. AMI'S VM> MVNY OTHK.lt (U'.NEUA THE KOS- 

 TIU'M ANH KHSTKOl.ATEUAL Ci'MIVVK I'MENTS 

 VKK O'Xl'KKSCENT. NOT SKPAKATE AS SHOWN 

 UE1JE I,!' 1 



of the compartments takes place at the basal margin and 

 usually at the sutures also, where the increment is at right angles 

 to that of the- parietes, and forms the radii. In species which do 

 not increase the diameter of the cono and orifice by the interposi- 

 tion of radii, the orifice is onlnrgc.il as the animal grows by wearing 

 away of the summit of the cone. 



FIG. 2. (1, CoMlVVRTMENT WITH TWO RA1UI. SERVING EIT1IEU AS A ROSTROI.ATERAI. OU AS A ROSTRUM 

 CONCRESCEXT WITH THE ROSTROLATERALS, AS IN BALANUS. 6, LATERAL OK CAKINOLATERAL COM- 

 rvKTMENT. C. CARUJ.V OR ROSTRUM. LETTERS AS IN fhi. I (.AKTEK 1>ARW1N). 



The upper ends of the pores or tubes in the parietes are more or 

 less extensively tilled up with calcareous material, occasionally differ- 

 ing in color from that of the rest of the wall. In the genus Tttraclitit, 

 inter alia, the outer lamina of the wall is ordinarily disintegrated 

 and fugacious in adults, leaving a columnar or tessellated surface, 

 formed of the harder tilling of the parietal tubes. 



The lateral edges of the radii are often provided with fine trans- 

 verse ridges or septa, usually having dentieiilate edges. These fit 



