12 



less different from the zooecial area, the aperture being especially of another form 

 or of another size than the zooecial aperture. As a rule it is more elongate, and 

 very often much larger, but in some cases it is much smaller and at the same 

 time of a quite similar form. At the whole there is seen similar differences in the 

 form of the aperture as are found in the avicularia of the Cheilostonmta. The 

 simplest form of heterozooecia is found f inst. in Mel. angulosa {pi. 11, figs. <S, 10), 

 Mel. Dollfiisi (pi. IV, fig. 23), Mel. sarissata (pi. II, fig. 1), Mel. punctata (pi. V, fig. 7) 

 and Mel. hexagona (pi. V, fig. 3). In the aperture there may be discerned between 

 the proximally situated "inner aperture" distally limited by a curved or angularly 

 bent line and a generally larger or longer distal concave portion, which corresponds 

 to the "oral ledge" of the zooecial aperture, and therefore must be designated in the 

 same manner. 



A sagittal section through a heterozooecium shows that the presence of this di- 

 stinctly bounded oral ledge is due to the circumstance that the inner distal surface of the 

 zooecial tube suddenly alters its direction forming an obtuse angle with the proximally 

 situated part, and the edge thus formed just makes the distal boundary line of the 

 inner aperture. In the heterozooecia of Mel. Filiozati n. sp. (pi. VI, figs. 7, 9) and Mel. 

 sqvamata (pi. V, figs. 14—16), and in the large heterozooecium of Mel. Roemeri (pi. 

 V, fig. 18) the oral ledge consists of a deeper median part and two more or less 

 developed lateral thickenings while in a larger number of species i. inst. in Mel. 

 magnifica (pi. 1, figs. 3 — 5, 7), Mel. lamellosa, Mel. pentagomim (pi. IV, fig. 22) and Mel. 

 gothica (pi. II, fig. 24) these thickenings have been developed into two inwards 

 sloping triangular processes, the "lateral processes" which conceal the larger part 

 of the oral ledge and the inner aperture. 



The thickness of these two processes (pi. 1, fig. 10) which may be coherent 

 distally decreases towards the median line where they are divided from each other 

 by a more or less narrow longitudinal fissure their free proximal margin being 

 divided by a transverse fissure from the distal margin of the suboral area. When 

 the state of preservation is not good the outer boundary of the two processes is 

 not distinct, and the opening formed by the two fissures may be taken as the space 

 once filled by the mandible. The aperture in all the species of this group is of 

 an elongate triangular form. A fourth group of heterozooecia which have been 

 found in Mel. Steenslrnpi (pi. Ill, figs. 11, 12), Mel. Cainii n. sp. (pi. Ill, figs. 19—27), Mel. 

 diirohrivensis (pi. IV, figs. 1-4) and Mel. Roemeri (pi. V, figs. 21 — 24) have that in 

 common with the first mentioned, that the aperture lacks both lateral thickenings 

 and lateral processes but these apertures have a form very like to or at least not 

 very different from the zooecial aperture, and most of them are, moreover, much 

 smaller than the latter. Also the heterozooecia themselves are as a rule much 

 smaller than the zooecia, and the smallest heterozooecia found in Mel. dnrobrivensis 

 only attain the sixth part of the length of the zooecia. A few of the heterozooecia 

 found in Mel. Camii have the apertuies provided with a comparatively narrow oral 

 ledge, but as a rule the latter takes up the larger part of the aperture and is per- 



