360 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1920. 



Some of the Trepostomata are incrusting and consist of one or 

 more superimposed layers, but most of them either rise into fronds 

 and bifoliate expansions or form hemispherical to rounded masses 

 of a size ranging as high as 2 feet in diameter. Such massive types 

 of zoaria arise from the fact that the zooecia in the Trepostomata are 

 directly superimposed upon one another to form the long tubes 

 which by continued budding result in the branching or massive 

 stony zoaria. These tubes are intersected by straight partitions 

 (diaphragms) or curved ones (cystiphragms), which represent the 

 covers and floors of successive layers. The diaphragms may be in- 

 complete or provided with a central perforation. As a rule, they 

 are few or wanting in the immature zone of the zooecia, but are more 

 numerous in the outer or mature zone, where also the zooecia are 

 often separated by more or less closely tabulated porelike spaces called 

 mesopores. Zooecial covers with a small subcentral orifice may 

 occur. 



One characteristic of the Trepostomata, which, however, the order 

 shares with the Cryptostomata, is the presence at regular intervals 

 over the surface of elevated groups containing cells differing from 

 the average in size (monticules) or spot-like areas (maculae) of such 

 cells on a level with the zoarial surface or depressed below it. The 

 size, shape, elevation, and distance apart of the maculae or monticules 

 are usually specific characters. The monticules may vary from small 

 sharp tubercles through rounded nodes to elevated rings completely 

 encircling the zoarium while the maculae, although often incon- 

 spicuous spots on the surface, are sometimes quite distinct solid de- 

 pressed areas or, as in one family, beautiful star-shaped regions. 



The spine-like projections on the zooecial walls called acanthopores 

 are found in thin sections to consist of minute tubes included in the 

 wall substance, but with a definite structure of their own. These 

 acanthopores traverse the mature region and undoubtedly represent 

 structures with some definite function possibly like the avicularia or 

 vibracular pores of the Cheilostomata. 



For many years the identification of the Trepostomata was based 

 upon external features such as the form of the zoarium, the shape of 

 the zooecia, and such surface characters as the tubercles or maculae. 

 This led to so much confusion that the order deservedly was not con- 

 sidered of much use in the identification of stratigraphic horizons. 

 The internal structure of these stony forms gives the true specific 

 characters, and so the preparation of thin sections for examination 

 under the microscope has become indispensable in their study. How- 

 ever, when once a species has been thoroughly worked out it can 

 generally be distinguished externally from associated forms of 

 similar appearance by quite constant differences which often seem 

 trifling and yet are doubtless of morphological importance. Even 



