13 



forated either with a small rounded or with a narrow triangular or fissure-like 

 opening. While the form of the apertures is constant in Mel. durobrivensis and 

 Mel.Steenstrupi, in Mel.Canni and Mel. Koeineri it is subject to a rather great varia- 

 tion both as to form and size, and in the latter species the largest of them are 

 ligulate and stretched beyond the distal angulale border of the zooecium. In Mel. 

 durobrivensis these helerozooecia are found not only interspersed among the zooecia, 

 but also constituting an incrusting base (pi. IV, fig. 19), and they seem here to play 

 a similar role as the corresponding kenozooecia of a Retepora-colony ^). 



In most species I have found the aperture of more or less helerozooecia closed 

 by a calcareous mandible (pi. 1, figs. 1, 2; pi. II, figs. 3, 16, 18; pi. Ill, figs. 3, 20; 

 pl. IV, figs. 1,23; pi. V, figs. 5, 8, 17; pi. VI, fig. 2) which has a similar arched surface 

 as the opercula and often shows more or less distinct radiating striæ. Sometimes 

 it is as many mandibles of cheilostomatous avicularia provided with a hooked 

 beak, being at the same time strongly arched not only from side to side but also 

 proximally distally (pl. II, fig. 3). 



In Mel. Caniii and Mel. durobrivensis the semi-elliptical mandible is provided 

 with distinct radiating striæ, and chiefly differs from the zooecial operculum in 

 being much smaller. Lastly it might be of interest to compare these helerozooecia 

 with those found in the Cheilostomnta, and in ordre to make the difference between 

 them more conspicuous we shall choose for comparison such presenting a maxi- 

 mum of outer likeness, f. inst. those found in a Thalamoporella-species-) and in 

 Mel. angulosa (pl. II). Besides the likeness in the form of the aperture we may in 

 both discern between an inner aperture and a distal concavity, but while the latter 

 in Meliceriiites is the distal inner surface of the zooecial tube it is in Thalamopo- 

 rella formed by a free lamina (a cryptocyst) which rises from the lateral and the 

 distal walls within the free margin. Besides the difTerence which the heterozooecia 

 of the Meliceritidae show IVom those of the Cheilostomata in being long slender tubes 

 the greatest part of which is hidden within the colony they present another con- 

 stant difference from the latter therein that the aperture is always limited by a 

 continuous calcified frame while in the Cheilostomata it is limited proximally by 

 a membranous area of different extent. The group of heterozooecia above spoken 

 of in which the larger part of the original aperture has been concealed by two 

 triangular laminæ shows a certain likeness to the heterozooecia of certain species 

 of Onychocella ') and Rhagasostoma in which the cryptocyst lamina has attained its 

 greatest development, being only provided with a small perforation for the occlusor 

 muscles, but the two laminae are two thick processes from the lateral parts of the 

 oral ledge, and the two mutually vertical fissures which may be compared to the 

 perforation in the avicularia of Onychocella and Rhagasostoma belong both to the 

 opercular area while the proximal part of the latter perforation is placed within the 

 suboral area. Lastly we shall remind of the different structure of the mandibles. 



') 18, p. 290, pl. X, ■-•) 18, pl. VI a. ') 7, pi. (i73, ligs. 1, 4, 8. 



